Bottlehead Forum
General Category => General Discussion => Topic started by: Larpy on March 14, 2024, 11:59:51 AM
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Hello all,
Last fall I was having trouble with vinyl playback. I started a thread that went 4 pages before I realized I had all the good advice I needed and it was time for me to troubleshoot on my own. No need to rehash that thread (https://forum.bottlehead.com/index.php?topic=15152.0 (https://forum.bottlehead.com/index.php?topic=15152.0), but in the course of trying to track down the problem, I created all sorts of new problems. As best I can tell, the original problem was a bad transistor (or a bad solder connection to that transistor) in my Eros. I now feel confident that I have fixed the original issue (and all of the collateral damage I created in trying to find that problem) except for one that I simply cannot figure out.
There is something "off" in my vinyl playback and the only non-objective evidence I have for it consists of asymmetrical waveforms I get when I record LPs. It is a weird problem whose fix eludes me.
Here is what I'm experiencing in a nutshell. The audio signal I get from playing an LP through my Eros to my preamp (routed through) the "preamp out" to a DAC to my laptop = asymmetrical waveform. See first attachment.
The signal I get recording a CD from my CD player to my preamp (routed through "preamp out") to a DAC to my laptop = symmetrical waveform. See second attachment.
Doesn't matter which LP I play or which input my analog signal is plugged into at the preamp.
OK, so you’d think there’s something amiss in my analog chain, but get this:
The signal I get from my turntable and Eros directly to a DAC to laptop = symmetrical waveform(!). See third attachment.
So there’s something wrong with my preamp? Apparently not. I swapped it out with my Moreplay and I got the same results as above.
What on earth could cause this?
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It's been a while since I looked at your initial thread, can you remind me of the cartridge being used and any step-up devices?
It is technically possible to overdrive the grid of the 6922 in the Eros, but you'd have to try really hard to do that, and it wouldn't disappear in your third measurement.
It is also possible to overdrive a Moreplay and the BeePre 2, but again you'd have to have an exceptionally hot amount of output coming out of the phono preamp. If this is all that's happening, then there are plenty of inexpensive and easy solutions to that problem.
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The cartridge is a Hana ML, which puts out .4mV. I'm using Sowter 1990 SUTs wired in the 1:10 configuration, so the signal to the Eros is 4V, at the bottom of the Eros' recommended input range. So that shouldn't be an issue.
The thing that baffles me is that the signal from the Eros directly to my Apogee Duet DAC interface is fine, but when that same cable is routed to either of my two preamps, where it gets routed from the selector switch to line level output jacks, the signal is asymmetrical. I've tried substituting a different cable from the preamp line level out to the Apogee, but I still get the same asymmetrical waveforms.
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It might help to get a scope and a generator and feed some small signals into the Eros/preamp combo to see what magnitude of voltage leaving the Eros has this clipping behavior. A cheapo USB scope and an app on a PC/phone/tablet with an 1/8" to RCA cable would be enough to get this done.
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I've never used an oscilloscope but I'm game to try. Does anyone have a recommendation for a cheap one? (I looked on Amazon and ebay and was bewildered by the variety.)
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If you are playing an LP do you need to run it through a DAC? If you were saying an ADC then I might think well OK. A CD being already digital doesn't need a DAC after it comes from the player either. I have a Behringer UCA202 for recording LP's. very cheap and there are different versions. See some of the workflows here. Most revolve round Audacity.
https://forums.stevehoffman.tv/threads/ripping-vinyl-to-laptop-computer-what-is-really-the-best-way.984126/
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I was a little sloppy with my wording. It's an Apogee Duet, so both a ADC and a DAC. It's the interface I use to digitize LPs. I mentioned it because it's the device that allowed me to see how my analog signal is, when routed through the line out jacks of both my preamps, distorted/asymmetrical. I use Vinyl Studio for recording, and that's where my waveform screen shots came from.
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I am wondering if you have listened to the recorded files? They obviously look like that something is wrong but how do they sound?
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Yes, I have listened to them. Whether the waveforms are symmetrical or asymmetrical, they sound a little harsh: no obvious distortion, but not the sound I'm used to.
Among the many odd things here is that, last fall, in a fit of frustration, I ordered another Eros kit and built an entirely new one (gave my other one to a young audio friend), and all of its voltages check out, so presumably it's not the problem. And just a few weeks ago, I schlepped my Linn LP12 to a Linn dealer (7 hours away) and had it serviced, and it sounded spectacular after he installed a few upgrades (never visit a Linn dealer with an LP12 if you're not prepared to be talked into an upgrade :)). So I know for sure it's not the turntable. The cartridge is brand new and I tried another brand new one as well to rule it out as the problem. In hindsight, I should have brought my Eros with me to the Linn dealer, but I didn't think of it.
I'm willing to learn how to use an oscilloscope, but I'm hoping someone will recommend one that's cheap but useful. I know absolutely nothing about oscilloscopes and every time I look for one online I get overwhelmed. I searched for "cheap oscilloscopes" on the diyaudio website, and the threads it yielded were themselves overwhelming, with many of the recommended oscilloscopes no longer available.
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I have not used one for low level.audio stuff, but the little $40ish digital scopes on Amazon have worked well for me for tuning oscillators and such.
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OK, I am now the proud owner of my very own pocket oscilloscope. I barely know how to use it, so please be patient with me, but I hooked up the oscilloscope the output from the Eros (RCA jacks) and looked at the voltage while playing an LP. The highest peak to peak voltage I saw was a little over 16 volts AC. Is that too high? Is that the source of my problem (bad sound and asymmtrical waveforms when I digitize LPs)?
The voltages inside the Eros all measure correctly. The cartridge's output is 0.4 mV and I'm using Sowter 1990 SUTs wired in a 1:10 configuration.
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To clarify: I'm still getting the same results as before: the signal from the Eros directly into the Apogee Duet interface (~16 volts AC p-to-p) is a symmetrical waveform.
The signal from the Eros to the Moreplay's tape out jacks to the Apogee Duet (~25 volts p-to-p) is asymmetrical. But the signal from my CD player to the Moreplay's tape out jacks to the Apogee Duet (also ~25 volts p-to-p) are symmetrical.
So if 16 volts from the Eros is enough to cause the asymmetrical waveform (clipping?), then why does the waveform look symmetrical when I digitize an LP straight from the Eros (bypassing the Moreplay and its tape out jacks)?
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It might help to get a scope and a generator and feed some small signals into the Eros/preamp combo to see what magnitude of voltage leaving the Eros has this clipping behavior. A cheapo USB scope and an app on a PC/phone/tablet with an 1/8" to RCA cable would be enough to get this done.
Here is what I tried: I played a frequency sweep through my phone's headphone out jack into the Eros input and recorded the Eros output through my Apogee Duet. I hooked one of the Eros output jacks to the oscilloscope and saw that (by adjusting the volume of the headphone out signal) I could modulate the amplitude of the frequency sweep's amplitude from around 1 vAC p-to-p on up.
The signal on the oscilloscope looked OK to me, but I admit I have no idea what I'm looking for--the same kind of asymmetrical waveform? If so, I didn't see that.
The waveform I recorded through the Apogee Duet interface looked symmetrical, but because it was just one (changing) frequency. I decided to play an a piece of music and repeat the process, recording the waveform as I turned the amplitude of the music track from 1 vAC out of the Eros up to about 16 vAC. Resulting waveform was symmetrical. See attachment below.
Not sure what I should try next.
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The highest peak to peak voltage I saw was a little over 16 volts AC. Is that too high?
That is, for lack of a better term, damn high! That would have to mean you have about 15mV going into the Eros, which would be a 0.4mV cartridge feeding the 1990 wired as a 1:20... When you listen to a recording, does it sound like the EQ is working properly? (that there is present bass and the treble isn't overwhelming?)
I would triple check the SUT wiring and the cartridge specs. If all else fails, then do your best to track down an old cell phone or tablet with an 1/8" TRS headphone jack and get an 1/8" to RCA cable, then we can send a very low level signal into the SUT and see what comes out.
You definitely have tracked down the source of the problem though, as you're overdriving the second stage of the Eros.
-PB
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Attached is a photo of the Sowter 1990 SUT wiring. It appears to be correct: according to Sowter, "for 1:10 operation connect the primary windings in series (Orange to Red)." That's exactly what I've done.
The cartridge is a Hana ML. Hana website identifies its output as 0.4mV.
I have an old laptop with a headphone out jack. I'm using a stereo 1/8" TRS jack to RCA jacks cable to feed the Eros. So I'm ready for your "if all else fails" option: what kind of very low level signal should I feed the SUTs?
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You will want to turn the volume all the way down on the laptop, then up the minimum amount off zero volume that you can. Plug that into your Eros (powered off is fine) and measure the AC voltage with your scope across the RCA jacks and then across the 47K resistors on the secondary side of the input transformer. This will allow you to verify the ratio and those transformers ought to take a fair bit more signal than the typical MC cartridge without a ton of issues.
If you see a 1:10 step-up, then let me know what the minimum voltage you have that comes out of the laptop and perhaps we can use it and your scope to check the EQ.
You may also find that this all works a lot better if the laptop is running off its battery.
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I'm back to troubleshooting after a few days of kitchen renovating. Lots going on right now.
The lowest waveform I could get on the oscilloscope from my laptop to the input of the Eros wasn't as low as I thought it should be: 99-114mV pk-pk (-3mV mean). At the other end of the Sowter SUTs I measure 1v pk-pk (mean bounced around from -4v to -15v).
This would seem to indicate a proper 1:10 step up of the voltage.
By the way, at the output of the Eros, I measured 6v pk-pk.
The next step?
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OK, 6V P-2-P is about 2V RMS, which is plenty low for the Eros.
I think it's worthwhile to to put low level tones into the input of the Eros again and measure the output. Try a 3.8kHz, a 1kHz, and a 300Hz tone. All the output voltages should be different.
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Got the sine waves from YouTube.
300hz at input 213-293mv pk-pk at output 13-14v pk-pk
1Khz at input 217-245mv pk-pk at output 7.12-7.32v pk-pk
4khz at input 126-174mv pk-pk at output 10.8-11.4v pk-pk
Way above the other day's 6v at output.
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300hz at input 213-293mv pk-pk at output 13-14v pk-pk
1Khz at input 217-245mv pk-pk at output 7.12-7.32v pk-pk
This is what I would expect to see. I chose the two frequencies that will be about half/double 1kHz.
4khz at input 126-174mv pk-pk at output 10.8-11.4v pk-pk
Why did the input level go down? That may be something that has to be looked at on its own. Still, you have a bit less than half of the input voltage that you had a 1kHz and you have more output than expected, which indicates that your EQ is not properly working.
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Did some more snooping of my Eros with my new 'sillyscope. I've concluded that the model I bought cannot detect AC signals below 100mV. That means there's no way for me to adjust the level of an audio signal from my laptop into the Eros that matches the .4mv output of my cartridge.
But my tests the other day seemed to confirm that my Sowter SUTs are indeed amplifying the signal by a factor of 10.
So I decided to assume that the signal entering my Eros was .4mv and the signal after the Sowters was 4mv. Can't measure either, of course, because they're too low for my 'sillyscope to detect.
I plugged my turntable into the Eros and played the first few tracks of the Cardas test LP: a 1KHz tone and a 10Hz-30KHz sweep, both recorded (mastered?) at –14 dBs. As expected, my little scope detected nothing at the input of the Eros and nothing after the Sowter 1:10 SUTs, but I got a clear signal at the outputs:
1KHz tone: 1.8v p-p
frequency sweep: 1v p-p from 10Hz to about 150Hz, where it rose to 1.5v p-p. Stayed there until 7KHz, where it jumped to 2v p-p. 2.9v p-p at 11KHz, 3.5v p-p at 14KHz. Then back to 1v p-p at 15K and then rising to 1.5v p-p at 20K. I have no idea if this is expected behavior or not.
Then I decided to play a regular LP and measure the output of the Eros. Highest measurement after a couple of minutes of watching the 'sillyscope was 6v p-p. Is that normal and/or expected, given my cartridge's .4mv output?.
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I would have a hard look at the RIAA, a misswired Or wrong component would mess up the eq.
Also if you need to continue it might help to jump out the transformers or injected the signal after. This would rule them out and you can raise your signal level up a bit, get it out of the weeds so to speak... John
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Why did the input level go down? That may be something that has to be looked at on its own.
I assume that's simply because I used test tones from different "channels" on YT. The first two I used came from the same YT source, but the third came from a different source that presumably put out a lower sine wave amplitude.
Today I'll follow PB's and John's advice and desolder, remove, and reinstall the components in my RIAA EQ stage. See if that makes a difference.
Thank you for your patience.
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Don't remove and replace EQ components unless they are determined to be in the wrong place. Just confirm.every position and reflow the connections first.
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OK, I'll exercise caution and restraint.
To clarify, I'm looking to measure the output of my Eros at 300hz, 1khz, and 4Khz, with the voltage at 300hz double that of the voltage at 1K and the voltage at 4Khz half of what it is at 1khz? Getting such results will indicate that my RIAA eq is functioning properly?
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I read that you are using youtube videos for the tones. Here is a website that will work a bit better. It is a tone generator and you can choose the output and frequency. That way all the volumes for the different frequencies are the same. Hope it helps. https://www.szynalski.com/tone-generator/
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OK, my latest update: I reflowed the solder connections in my RIAA network and got the following measurements at the Eros' output jacks using sine waves from the site Stefan recommended (thank you for the recommendation Stefan; it's a great site and much easier to use than YT).
300Hz 8.7v p-p Right channel; 7.7v p-p Left channel
1KHz 4.7v p-p Right Channel; 4.1v p-p Left channel
4kHz 2.37v p-p Right channel; 2.0v p-p Left channel
These measurements were taken with a feed from my laptop with its volume set as low as it would go and the volume output at the tone generator site set at 9%. If I turned the volume of the tone generator any lower I couldn't get a reading at 4KHz on the oscilloscope.
I played a sine wave and checked the difference in AC voltage between the input to the EF86 and the output jacks and in order to get a measurement on the oscilloscope at the EF86 I had to turn the volume of the input signal up quite a bit, but what I measured was 574mV p-p at the EF86 grids and 102 p-p at the output jacks. Is that expected behavior?
And that reminds me: I've been regularly injecting far larger AC signals into my Eros than it would see from playing an LP. Could that harm the circuit? The Sowter SUTs? I checked the resistance across their primaries and secondaries and they still measure in spec.
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The issue with signals that are 100x what is intended is that you are looking at severely distorted (clipped) waveforms to get your voltage readings since the 6922 will be way overdriven. The results will not be very accurate. Sorry for giving a recommendation for a cheap scope that apparently doesn't have the needed sensitivity. I have only used mine for automotive and motocycle stuff, not low level audio signals.
That said, the result does sound like the preamp is amplifying properly. It could be that the low signal level you get once the Eros is put back in the system is a problem somewhere before or after the Eros. Maybe a cabling issue from the tonearm/cartridge or the wrong input setting on the Moreplay.
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Can a 6922 tube be damaged by being overdriven the way I have been doing?
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Hooked my Eros back up to the audio system and, unfortunately, I'm hearing the same distortion on loud passages I've been hearing for months now. I recorded about a minute of an LP and once again got an unsymmetrical waveform. See attached.
I watched my little 'sillyscope while I listened to the LP and the massed strings crescendo that sounded distorted measured around 9v p-p.
So I'm afraid I'm back at square one. Assuming a 9v p-p signal doesn't indicate the Eros is being pushed into overload, that is.
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Would hearing an audio file help? Is there a way for me to share a brief (40 second) audio file of me switching back and forth between the same music on LP and CD? The difference in sound quality is dramatic. I could also attach the waveform of the recording as captured on Vinyl Studio so you can how LP waveform is asymmetrical whereas the CD one is not.
I'm technologically backwards enough to have no idea how to share a brief MP3 file with others on a forum.
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OK, I'll drop the audio file idea.
Today I tried bypassing the Sowter SUTs and listening to (and recording the waveforms of) an LP. Had to turn the preamp volume up all the way and there was tube rush, but the LP seemed to sound distortion-free. What’s more, the resulting waveform, though tiny and hard to magnify clearly, looked symmetrical.
Is that a clue? Could the asymmetrical waveforms I’ve been seeing and the distortion I’ve been hearing be caused by the SUTs stepping up the signal from my cartridge too much?
If so, this wouldn’t make sense: I have a cartridge (Hana ML) that puts out 0.4mV into Sowter 1990 SUTs wired 1:10. That should put the input to the Eros circuitry at the low end of its 4-6mV target range. And yet when I’ve recorded music from LPs in the past few days, the signal into my digital interface has clipped a few times. That means the Eros is putting out a signal somewhat hotter than line level, yes The badly distorted mass strings I heard the other day looked to measure 9v p-p on the oscilloscope. From what I understand, that shouldn't be too hot an output signal from the Eros.
As far as I can tell, my Eros is wired correctly. The oscilloscope measurements of my RIAA network now read as expected. I’ve checked and rechecked my wiring of the Sowters (and I double-checked that they are indeed 1990s). And yet could the AC signal into/out of the EF86s be too high? Is there a wiring mistake associated with the 6922 tube that would explain my symptoms?
I’ve swapped out all of the tubes and that makes no difference. Changed interconnects. Surely a bad AC cable couldn't cause this. Right?
I’ve been reading up on asymmetrical waveforms and, from what I’ve gleaned, DC offset can be the source but when it is the mean amplitude will be displaced from the central axis. That doesn’t seem to be the case with my asymmetrical waveforms, so I’m thinking DC offset is not behind my problem. From what I remember observing my oscilloscope while I play music through the Eros, my mean amplitude is typically –1 volt or so. And the waveforms from music I’ve recorded on Vinyl Studio look on-axis.
If I correctly understand what I’ve been reading, an asymmetrical AC waveform where the mean amplitude is on-axis (or close, as in my case), the source of the asymmetry can be high pass filters or “aggressive low-end processing.”
Is it conceivable that I’ve somehow inadvertently created a high pass filter in both channels of my Eros? What kind of miswiring would create such a high pass filter?
This is the most puzzling and frustrating problem I’ve faced in 20-25 years of tube DIY builds. And I’ve done dozens.
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Somewhere in the thread so far there is a statement that the voltages are all within spec. But some of the other observations make me wonder if the 6922 is not operating correctly. If you can measure the DC voltages at the pins of the 6922 (pins 1-3 and 6-8) that would help. The correct voltages are only a little different from incorrect ones, so it's easy to mis-interpret them.
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Measurements on the 6922 tube:
pin 1: 160vDC
pin 2: 97vDC
pin 3: 98vDC
pin 6: 160vDC
pin 7: 97vDC
pin 8: 98vDC
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That's pretty close to the design target, though I'd rather see 170v at the plates. The difference means your 6922 has a slightly higher transconductance than nominal, which is generally a good thing but can upset the operating point a bit in this direct-coupled circuit..
If you have another 6922 on hand, preferably a well-used one, it might be worthwhile trying it. If not, you can measure the voltage difference between cathode and grid (pins 2-3 and 7-8), which should be about 1.50 volts. I don't expect that to show a problem, but our expectations have been a poor guide so far!
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I don't have any well used 6922s other than the Bugle Boy 7308 I've been using for a couple years.
Difference between pins 2 and 3 on it is 1.5 vDC. The difference between pins 7 and 9 is a mere 1 volt.
I have a reputedly NOS Siemens 6922 that measures 160 and 155vDC on their plates, and
the voltage between pins 2 and 3 and 7 and 8 is a bit below 1.3 vDC in both cases.
The Russian tube that came with the kit measures 162 and 156vDC on their plates.
The difference between pins 2 and 3 is only .7 volts. The difference between pins 7 and 8 is a paltry 0.55 volts!
I'm consistently low with all three tubes. Might this be an important clue?
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It is indeed - those grid-cathode voltages are too low for proper performance.
The first thing to check is the 47K/2W resistors - are they the right resistance? Are they properly connected? You should be able to measure the resistance from the plates (pins 1 and 6) to ground, with the amp power off.
While you are at it, also check the resistance from the cathodes (pins 3 and eight) to ground, which should be 27K - this will take some time, as the 100uF/160v caps will have to charge up. Also confirm that those caps are properly oriented.
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The resistance from 6922 plates to ground is 44K in both instances. With the power off. I checked the 2 watt 47K resistors on the board above the 6922 socket and they are the correct 47K resistors. Just to make sure, I desoldered one leg of one and measured its resistance out of circuit: 47K Ω.
I reflowed the solder connecting those two resistors but no change in resistance.
Am I right in guessing this suggests something is miswired elsewhere and pulling the 47K Ω resistance down about 10%?
Measuring the cathodes to ground proved easier if I disconnected the bypass cap (which is not the stock electrolytic but a Panasonic DC link cap and so not directional). Resistance from 6922 cathodes to ground is 27K Ω on the nose.
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I hate to say this, but things are pointing to that 100uF bypass cap. The circuit is behaving as if something is draining current in parallel with the 27K resistor, and the only candidates I can see are that cap or the EF86 screen grid.
To test this, with the cap disconnected, repeat the voltage measurements, including the grid-cathode difference as well as the plate, grid, and cathode voltages.
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Don't hate to say anything! Any advice you offer is immensely appreciated, and I'll be ecstatic when you find my problem. I haven't been able to listen to LPs in 5 months, so at this point I'd happily replace any component to get my Eros happy again.
Measurements with 100uF caps disengaged (they actually measure 110uF):
Pin 1: 162v
Pin 2: 98.8v and then drifted down. After a minute or so it was down to to 97.7v
Pin 3: 99v
Pin 2 to Pin 3: 1.1v
Pin 6: 157v
Pin 7: 101v
Pin 8: 101.3v
Pin 7 to pin 8: started at .4v and slowly rose (over a few minutes) to .85v and then began drifting back down. After a couple of minutes it was .76v. Now that is weird.
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This is a real puzzler, all right.
At least we've found a symptom that explains the one-sided clipping problem. The 6922 bias is too low and it is going into grid current, clipping one side of the waveform. The problem is, the voltages and currents don't make sense
The only idea I have left is a possible radio-frequency oscillation problem. Those are usually dependent on layout and lead dress, so any non-standard component or change in wire routing becomes a suspect. Can you locate the original 100uF caps and install them on the PC board as shown on page 70 of the manual? Leave the film caps detached from the cathodes. Are there any other deviations from the stock configuration? They may have to go, too.
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I don't seem to have any 100uF electrolytic caps in my cap drawer. I'll order a couple from Mouser. If I remember right, they were Lelons? This is probably a question for Doc: have Eros kits been shipping with Lelon 100uF/160v caps recently? I'll play it safe and order a pair of Lelons and a pair of Nichicon UCY caps--they're rated for 12,000 hours.
I'll report back once the caps have arrived and I install them.
Here is the complete list of the other non-stock components in the Eros:
• SBE 716P "orange drop" caps in the RIAA network
• Auricap caps as the output caps
• 0.1uF Russian K40Y-9 caps on the voltage regulator board
• Dale CPF 2 watt 47K Ω plate resistors for the 6922 tube
• Vampire RCA input jacks
The big Panasonic caps are the only component that required non-stock lead dress.
The interesting thing is that most of these components were imported from my first Eros. I used the Panasonic, Auricap, and Russian caps in my first Eros for a year and a half with no problem. Quick recap: I happily used an Eros for a year and a half when, last fall, it suddenly started sounding distorted. My attempt to identify the problem turned into a nearly comical creation of new problems, but I'm 99% sure the original problem was a flaky transistor solder connection. But I got so frustrated I ordered a new Eros kit during last Thanksgiving's BH sale. I put this new kit together with all new parts except for the Panasonic and Auricap caps--they came off my old Eros and were installed in my new Eros.
I gave my first Eros to an audio friend, but he's out of the country until later this month. Otherwise, I'd have him come over with my old Eros to compare.
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100uF caps arrived today and I soldered them in to the board above the 6922 tube socket as shown in the instructions. I put the Bugle Boy 7308 tube back in (it seems the happiest of the ones I have for duty in that socket) and the measurements looked better:
pin 1: 167v pin 6: 161v
pin 2: 94v pin 7: 95v
pin 3: 96v pin 8: 97v
pin 2 to 3: 1.6v pin 7 to 8: 1.3v
I hooked the Eros back into my system but unfortunately the distortion is still there. And the asymmetrical waveforms.
I guess the next step is to remove all the other non-stock components I've used. Since I was ordering the 100uF electrolytics, I also ordered replacement Dale resistors that get soldered to pins on sockets A-C (the same ones that came with the kit). I think I'll rebuild all three sockets and their connections and go really, really slowly.
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I don't have a better suggestion at this point. I wish I did!