Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Eros Tape => Topic started by: mkontor on August 20, 2013, 06:29:00 AM
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Well it all works seemingly as it should but I some issues:
there is a hum that is present even with the inputs shorted; no earthing arrangement does anything, and I know a bit about hum - it is internal to the amp
the ef86's are quite noisy - lots of tube rush (wind) and they are quite microphonic; I have the the Svetlana's supplied, some Siemens and Mullards.
The worst thing is the sound - too bassy and not enough top end, very rolled off. Far too 'tubey', not very transparent. I can't believe this is how bottlehead wants this to sound, so I am hoping something is wrong with my work.
Have owners found a significant difference in overall tonal balance compared to their original decks (using the built in preamp)?
I don't mean, it's sweeter, more fluid, etc etc, I mean only in terms of tonal balance.
Cheers,
Mike
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Need a lot more info, but based upon your description I would guess that the playback head output is relatively low, and you have cables between the head and the preamp that have high capacitance. What is the deck?
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The worst thing is the sound - too bassy and not enough top end, very rolled off. Far too 'tubey', not very transparent. I can't believe this is how bottlehead wants this to sound, so I am hoping something is wrong with my work.
How did your voltage checks go? Are you using the phono Eros or tape Eros?
Generally, a frequency response anomoly like this would be the result of a mistake in constructing the EQ in the circuit, though there are other potential sources.
-PB
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All voltages check out fine.
It's not a cable problem as this hum exists with the inputs shorted, as I mentioned....
Using a Pioneer RT707 with relapped playback head (forward direction only) with very well shielded cable to gold RCA outputs
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I tested the unit with my turntable (Grado MM cartridge) and the output is higher but not significantly so. I know the EQ is different for phono but even so, the same sonic signature was there - bassy and rolled top end. It's not there is NO treble, but there is no sparkle, and the roll off is significant. The standard electronics in the Pioneer are better in every way, at this point.
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in fact the cable from the head are very low capacitance designed for phono lead (can den Hul D502II hybrid), and anyway high capacitance would not explain the bass boost.
I don't see how I could have got the installation of the EQ section wrong...it's a simple kit (I have assembled full blown amps in the pastt and make loudspeakers)
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in my previous reply I meant to write
' It's not that there is no treble....'
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I don't see how I could have got the installation of the EQ section wrong...it's a simple kit (I have assembled full blown amps in the pastt and make loudspeakers)
Nobody is above making mistakes!
Can you tell me the OB and Breg voltages on your PC boards?
Can you triple check that the 0.47uF and 0.047uF caps are in the correct position?
Having measured plenty of tape Eros units, the frequency response is quite flat and tipped up bass indicates an operational issue (or potentially a tape head issue, but you would also have this through the stock electronics).
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I did not mean to imply I was perfect (LOL) in fact I did make two mistakes which as you see in previous posts were rectified. But I have checked the construction and measurement many many times. Still, I am hoping for a mistake.
ALL voltages are exactly what the manual says they should be KREG is 1VDC breg is 100VDC, IA is 225VDC
Yes the caps are in the right place.
Any ideas about the hum?
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On page 62, there is an error - it says to mount the resistor from T111 to T10 - it should say T12, as shown in the picture; this was my first mistake, i followed the instructions then got an incorrect resistance check on that channel (see another post)
On page 63 of the instructions you show 2 pictures; the bottom one shows two components on each side that are not in any of the other pictures, nor are they mentioned in the instructions; these are
a cap from T11 and T12 and another from T4 to T5
a blue resistor from T12 to ? picture is cut off and another from T4 to ?
I think this is the EQ part of the circuit so could be the cause of my unflat frequency response
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more about the caps shown on page 63 - these are 630V, and there are these in the parts list:
( ) 2 - 0.1 uF 630v metalized polypropylene capacitors ( F104K) or (0.1uFJ)
( ) 2
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I'm not yet at the office, but when I get in I will check into those photos for you. It's been a while since I had my head under the hood of a tape Eros, so my recollection of the parts list is a little foggy and I need to get to all my documentation so I don't further confuse things. It is possible that we were trying to focus on some other aspect of the assembly in those photos besides the parts in question and we may have put a detail shot from a different preamp (i.e. the phono version) in because it was more clear. If they are only visible in one or two photos but not in the photo of the completed underside that is probably the case.
Just a few ideas in the meantime that might or might not help -
Re the hum, the circuit is very sensitive to magnetic fields from other pieces of gear. I have had a couple of occasions where 60Hz hum that seemed to be coming from an Eros itself was actually due to its sitting too close to the power transformer of a tape deck. This hum can be present with inputs shorted as it is coupled into the circuit components themselves.
Also you mention that hiss and hum are both present. That makes me wonder if the preamp you have the Eros connected to has a lot of gain. If the head output is relatively low (like under 2mVrms) it could be that the volume control needs to be turned up a lot and that your preamp is capable of amplifying the low noise floor of the Eros, which when greatly amplified does have a slight hiss component from the EF86s and a very tiny hum component. These aren't usually audible unless the volume is cranked way beyond comfortable listening levels, hence my wondering about the play head output level. Does it seem that you need to turn the volume setting much higher using the Eros than if you use the inboard electronics of the deck?
I'm not saying that either of these is the actual issue, it does sound like maybe there is some fault in the EQ circuit. But it's worth eliminating these things as possible contributors to the problem.
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It does indeed appear that the photos on the bottom of p63 and top of p64 are actually from the phono version of the manual. Anything other than the wires called out in those photos can be disregarded.
Have you tried substituting some different cables between the playback head and the Eros? Rolled off treble and tipped up bass are basically the same thing, as there is a limit to the amplification factor of the Eros and thus all EQ is really only subtractive, i.e., there is no real bass boost, only mid and or treble cut. Hence the influence of a cable interaction that rolls off the treble can seem like it also boosts the bass. I don't mean to seem stuck on this point and I am certainly willing to look at every possible reason for the issue. But I have had to deal with this exact issue at more than one trade show where an exhibitor has used an Eros with a high quality cable of their own choice and complained that the treble was rolled off, only to find that when we changed to a different cable everything was fine.
I am also not familiar with the head in an RT707, so I can't know for sure if its impedance is a good match to the loading we provide. That loading has been optimized for heads we are familiar with - Technics RS1500, Otari MX-5050, Studer, Ampex ATR, Nagra T Audio, Flux Magnetics extended response. Those for the most part are around 200mH inductance and a load of around 75K to 90K seems to work well. It is possible that a different load impedance could help a bit, but without any specifications it would have to be a cut and try type of adjustment. It's possible to replace the input load resistor with a 100K or 250K trim pot wired as a variable resistor to make this adjustable.
Of course all of this is long distance speculation. To best solve the mystery a response curve of the preamp would be the first, best step.
OK we have discussed checking the EQ to make sure it is correctly wired and that the caps values are correct. We have discussed head to preamp interconnect cable and head loading. We have discussed possible coupling of hum from outside sources. It could also be a tube issue, though that is typically a problem in one channel, no both. Substituting a different set of tubes might give some information. Also, the output impedance of the Eros is about 3K. If the load it is running into is very low, like 3k or less, or if the cables from the Eros to the next piece of gear are very long that could have some influence upon the perceived bass performance, i.e. it could sound loose or rolled off on top. If the load is above 10K or so and the cables are under 2M long this is probably not the issue.
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thank you for your long reply
when I said noise I meant to rush not hiss.. the preamp is quite noisy ... I did not have to turn the volume up very much at all... it's about the same same thing as direct out... the cables i used are standard cheap ones because I did not want to make good ones up yet.. anyway I am familiar with off with cables and this is certainly not the case this roll off and bass boost is quite severe... I sent a unit to the guy that makes my valve amplifiers is to check out so I don't have it with me at the moment but obviously if he gets hum as well there is a problem with the unit.. He designs valve amplifiers and is very knowledgeable
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sorry was using phone to type and there are some errors in my reply
I meant to write tube rush nit his.... and that I sent the unit to my friend.... the other errors I think you can figure out
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I have tried 3 brands of tubes all the same results... I have to say that the ef 86 is not a great tube.... there was a big difference in tube knows when swapping tubes left and right hand and between the brands
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could you let me know where the eq circuit is and where we could install some pots for the bass and a trebke section
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I hopped up my turntable with standard moving magnet cartridge and I had to turn the volume down just a little bit so the output of the pioneer is not that low.... of course the eq is different for phono but I still heard the same treble roll off and bass boost
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Trying phono was a good idea. The EQ section is between the two stages. Your designer friend should be able to figure out the resistors to replace from the schematic supplied with the kit. But unless the parts are in the wrong places or for some reason the values are wrong all that changing to pots will do is take the curve away from the proper turnover frequency points. The accuracy of the Eros EQ curve is generally regarded as quite good compared to other products and thus it would seem there is something that is not the way it should be like an out of spec part or a miswire.
There seem to be a couple or three different things going on here and consequently I am a bit puzzled as to what the cause is. We would be happy to have a look at the preamp for you, measure the frequency response of the EQ circuits and check the function of the EQ and also look at the noise floor, and correct any issues free of charge.
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TX fitter all that, to save on shipping costs I will let my designer friend do this... it would be helpful helpful if you could tell me some short cuts like where to measure and what to measure... remember he does not have a test tape... however you can certainly check out everything else... I'm really hoping I have done something wrong because this unit really does not sound correct
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The service manual is available online from "hifiengine"; I registered and downloaded it. The tape head preamp assembly has a very-low-frequency gain of about 1800 (about the same as Eros) and an input impedance of 100K, so I speculate the tape head is similar to others for which the Eros is designed. The equalization on that board looks to be similar as well. The head itself is part number RPB-041, but I can't find any specs. There have been a few threads on AudioKarma - I didn't see anything useful though.
Based on the above, I would expect a correctly-functioning Eros to provide nearly inaudible tube rush and an excellent frequency response, so long as its input is correct (the correct head coil, direct to the Eros and disconnected from anything else - there must be a switch in the deck for bi-directional operation?) and its output goes to an appropriate load. All these have I think been mentioned, but one at least of them still must be the culprit.
Be sure to give your friend the manual, which has the circuit as well as the pictures showing parts placement. The circuit has terminal numbers identified which is a big help.
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Thanks for the offer to look at and fix the unit. I will let my friend have a go at at least finding the source of the hum and noise. He will also check the construction. As you say in your manual, a second pair of eyes...
He does not have a tape deck with direct head output, so will not be able to tell much about the sound, however you get a fair idea with a MM cartridge (turntable).
If he has no joy I will get him to send it to you.
As far as my Pioneer goes, the forward head has been directly wired to a pair of RCA sockets with extremely well shielded cable. It is not connected to the internal amps when I've tested the Eros.
Cheers,
Mike
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When hooking up a turntable with MM cartridge, what should we expect to hear in terms of sound (leaving the EQ in the Eros unchanges , with NAB selected) - more or less treble than with tape, more or less bass than with tape?
I'm asking because my tech friend does not have a tape head output....
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The treble turnover frequency for RIAA phono EQ starts a bit lower than NAB. So the treble from an LP will sound slightly bright through the Eros tape head preamp.
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Well that is certainly not happening now, so there is definitely something wrong with the EQ...
I'll keep you posted....
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As an aside, what are the main differences between the Eros and your $4000 tape preamp? Going from the description, it's the same design.
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Hi Paul,
When you say the manual - is this the same as what came with the kit, because that has circuit diagrams in it as well....
Out of interest, my tech friend (Elson Silva, he builds the Cymer amplifiers) really likes the 5775 tube as an input tube. If we get the Eros up and running as it should, we will try this tube to replace the EF86. I have a pair of NOS Siemens and Mullard EF86's and one of each pair is very noisy (this is not the tube rush I refer to - it's there on the quiet tube; BTW it is there with the inputs shorted, as is the hum), so I am not a big fan of the EF86, so far....Elson says the American EF86's are much better mae.
the quietest ones were the ones supplied (Svetlana), but swapping the pair left with right make the left channel more noisy.
Cheers,
Mike
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Paul - I misunderstood - the service manual you are referring to is the one for the Pioneer; I have this.
Anyway, the tape head is directly wired to gold RCA sockets; the forward head has been relapped, the reverse, erase, and record heads removed to provide a better tape contact with the forward head. IMO this deck sounds superb - it certainly beats vinyl in every way, with the best tapes (and I have some wicked vinyl and a great t/table).
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The Tube Repro adds a trim-able transformer balanced output stage at +4dB, premium parts like Teflon caps Tantalum resistors and turret boards along with a rather spendy cabinet to house it all in. It's hand wired by PB.
I will let Paul weigh in on changing the input tube from an EF86 to a 5755 if he so desires. The circuit is direct coupled and thus there are a lot of interactions to consider in making such a change. A much less complex mod is to rewire the socket to use a 6BR7. However I think this getting way ahead of the game. The preamp should be verified to be working to spec before mods are done. Unfortunately we are a bit restricted in being able to help verify that unless we have the opportunity to look it over.
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I agree it's getting ahead of the game - just wanted to mention it for your interest and because I was getting poor results with noisy EF86's.
I may know the source of the hum - the isloation transformer that powers all the plate amps that drive my bass speakers has a hum in it that on all the sources is very low in level, but as you say the Eros is sensitive to picking up hum. One thing I did not try is to turn this isolation transformer off or plug the Eros into another power point in the rooms (all the other gears including the Eros is plugged into a totally silent, better quality isolation transformer.
Elson has the unit now and if he gets no hum, that problem is solved.
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I can't locate any data on a 5775.
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It's a bigass, obscure rectifier. Hence I am pretty sure he means 5755. Kinda like a 12AT7.
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Yes I think I remember the number wrong, only came up briefly on a conversation about EF86
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Take a look in the Eros Phono file. I have a thread listing the drop in replacements/equivalents for the two audio tubes in the Eros. My particular favorite are the USSR Military surplus tubes. They are more rugged than other designs. That is for both the EF86 and the 6922.
They also tend to be quieter as well. The ones from the 60s and 70s are reported to be the best. Most of the sellers will send you what you ask for.
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Just to keep you updated, the Eros got lost on the way to my tech friend, hence the delay; it's on the way back to me and then I have to send it to him again, so I won't have anything to report until late next week.
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I would suggest that you send it to Bottlehead instead. They know the circuitry, have tape decks to test with therefore they can be more efficient locating your problem.
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There seems to be a bit of this going around. I got call from a tech in Texas yesterday (Pete Nyquist, super nice gent!) who had been sent a deck we modded a couple of years ago. He told the client it should have gone to us because we designed the mods done, and when he called me and I concurred he already had it packed and just wanted our address.
I'll just suggest that perhaps the return of the shipment is a sign from the cosmos...
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Hi Dan
I've got the unit back now and thought I would check it out again before sending it to my tech and found that in the process I have lost the instruction manual which I had copied to my laptop, which I had to restore, so lost the file. I also lost the CD!!!!!
Can you please send me the PDF? I have dropbox ([email protected]) or please burn an image and point me to it? It will take too long if you post me a new CD.
Meanwhile I measured the resistor in the EQ section and they are both 200K, from memory that's correct. Did some other measurements too, all OK (from memory and more to the point, equal in both channels).
The hum is still there, fairly low level, but my tech is an expert at making tube amps with no hum at all.
I hooked up my LP player again as my Pioneer is wired up as stock again and it's a pain to re-wire, and the sound is definitely too warm, lacking in bite and treble. It's much brighter, has more attack thru the phono stage, so there is something amiss...
Without the instructions or at least a circuit my tech will not be able to do much...
Cheers,
Mike
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Dan
I am going to send this to you. It's easier. You know this model.
Cheers,
Mike
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I promise we will give it a full exam and make sure it functions to exact spec.
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I don't doubt that one bit!
Additional to my note in the box, I think it might be wise to fit a 110v transformer as the one that's in there runs very hot, and I can use a step down transformer, not a bad thing as it provides some filtering.
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Could you please somehow send me the instructions as I do seem too have lost this...
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Additional to my note in the box, I think it might be wise to fit a 110v transformer as the one that's in there runs very hot, and I can use a step down transformer, not a bad thing as it provides some filtering.
The transformer voltage has no effect on the heat; the lower voltage transformer takes twice the current so it makes the same heat.
And yes, the Eros transformer does run fairly hot. That's because it has a lot of turns, to minimize the stray magnetic field and any mechanical vibration or buzzing.
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Well actually I knew that, somewhere in the back of my mind....
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Dan et al - I have found the instruction CD so no need to send again
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Dan
The Eros had arrived at yours...
Mike
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PB should be in the lab today and will have a look at it.
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I would prefer to be contacted by email michaelkontor (at) bigpond (dot) com rather than on here.
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Dan - no progress yet?
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We are incredibly busy this week, but PB has been working on it. There was some shipping damage, it appears that a part or two is somewhat melted from the soldering iron during construction, etc. and he is working to get it into condition to be able to test it.
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Just let you know I was able to measure the pioneer head...impedance is 250 at 100Hz, inductance is 170....mike