Eros Phono finished! Plus question about gain

[Grainger49]

Dave,

I kind of think that you have swapped resistors somewhere.  My Eros has copious gain.  My seduction replaced an ARC SP-14, big brother to the SP-9. 

You might start checking the resistance across the resistors and checking against the values called for.  You won't be the first builder to swap out a pair of resistors.

[dave-tx]

Hi Grainger - Double checked all resistors, and to the best as I can tell they are all in there correctly.  All transistors appear to be in the right direction as well.

My logic seems to say that if I have the same issue in both channels then it's not likely to be a poor solder joint or single bad component, because in the case of a gain issue, there would be a mismatch between channels if that were the case.  Would you agree?

Looking at the schematics, the only way I can rationalize my resistance measurements is that I am not seeing the ~500Kohm resistance presented by Q2+R2 in parallel with the T2, T4, T9, A6, C1, C2 (and the corresponding B side) measurements.  The only conclusion that I can draw from this is that my MJE350 transistors are backwards.  I have the metal plate side facing out, as per the manual.  Is this possibly incorrect for the particular transistors I received?

I'd like to get a second opinion before I desolder and reverse them.  (as an aside, what is the functional consequence of having these backwards?)

EDIT: thinking about it more, I don't think these could be reversed and have any function at all.  I'm still looking at this thinking that my measurements aren't taking that path into account.  What does that mean, I don't know.

Thanks,
David

[Paul Birkeland]

Do not reverse the MJE350's, you will do a great deal of damage to the amplifier.

Please pass a 60Hz tone through your preamp and measure the output that you get from the various inputs, I stil stand by this being a very important step in the direction of diagnosis.

-PB

[grufti]

David's amp is the Eros. There are no various inputs, there is just one. A 1kHz tone is much better suited for his testing than a 60Hz tone.

[Paul Birkeland]

This was in reference to his Counterpoint preamp, which has several different inputs padded down at different levels of attenuation.

A 60 Hz tone is measureable with a multimeter, hence my recommendation.

-PB

[dave-tx]

Ah yes, thanks for reminding me.  I haven't measured the preamp yet, but did some measurements on the Eros itself.  I think these results point more to a problem in my build than an issue with the preamp.

I hooked up a few test signals, here are the results.  I used a signal generator on an iPhone and measured with my scope probes.  Don't know if I needed to match any particular output load(?)

All measurements peak-to-peak, 20mV input

60Hz:   1.3V out
500Hz: 0.5V
1kHz:   0.35V
10kHz: 0.30V
20kHz: 0.22V

Both channels measured approximately the same.  It appears that even the 60Hz level is below nominal.  Or am I fooling myself because I'm not using an inverse-RIAA source (although 1kHz should be the 0dB point on the RIAA curve, yes?)?  Sorry for the clueless questions, I'm learning as I go.

Are there any good internal nodes to measure to try to track down the missing gain?

Many thanks for the continued help.
-David

[grufti]

The ratio between your 60Hz measurement and your 1kHz measurement is close enough at ~ 11.4dB. That difference is the result of the RIAA filter doing its job. This tells us that your meter is reasonably accurate at 1kHz, but it's still worthwhile to find out what type of meter you used for the measurements.

That being said, you should measure way more than 0.35V at 1kHz with 20mV in. The Eros should be right around 1.5V out with 5mV in. Something inside your Eros isn't working the way it should.

20mV is a bit much on the input for your measurements [5mv would have been better], but in this case it is definitely good enough to point to the culprit.

[dave-tx]

Good that the ratio between the frequencies seems right.   I was just using an old Techtronix oscilloscope to measure both the input and output signals, probes set to 1x.  So my readings involve some visual interpolation between grid marks, but they should be reasonably accurate.

I was initially using 5mV in, but 5mV/div is the smallest my scope will go, so wanted a little more resolution.  I will drop it down to 5mV input for any future measurements...that's closer to what my cartridge should be putting out anyway.

Thanks,
David

[grufti]

Your measurements at 10kHz and 20kHz are well below what they should be. 20kHz should be almost 20dB higher than the 1kHz value with the RIAA eq, but that issue is probably tied up with the underlying problem with your Eros. I would not worry about that now.

Your scope won't have any issues with measurement accuracy at frequencies higher than 60Hz. That's good to know.

PB will know where to look. He knows way more about the BH amps than I ever will.

[Doc B.]

Are you using the same units for input and output measurements? Vrms?

[dave-tx]

Are you using the same units for input and output measurements? Vrms?

Pretty sure they are the same - It's probably 15 years since I've used my oscilloscope, but I believe both scope channels are set up identically.  My measurements should be peak-to-peak, since I'm "measuring" by visually looking at the waveforms.

Thanks,
David

[grufti]

You should poke around socket C in the middle and also check your 6922, E88CC, 7308 tube since both of your channels seem to have the same problem. Unfortunately the wiring is rather crowded right there. Check the continuity of your ground connection(s) in that area first (ground near socket C to input ground for example).

Check the tube itself and/or replace it.

Check all resistance measurements and voltages for the connections to socket C. It gets pretty crowded in there, but you can follow the wiring to the next terminal strip. You can use page 1 of the schematic to find your way around. The socket pins are shown in the schematic: C1 and C6 should be at bit less than 170V, C2 and C7 at about 100V, C4 and C5 are the heater, C3 and C8 are the cathodes. I don't remember off the top of my head what the cathode voltage should be. There are references to the corresponding terminal strip points in the schematic. What ultimately matters is what the tube sees at its pins.

Be careful measuring directly at the socket. There isn't much room for error. You probably want to connect your probes while the amp is turned off and then turn it on to measure.

[Paul Joppa]

20mV pk-pk is 7mVrms, a very good number for testing.

350mV/20mV (gain at 1kHz) is 25dB gain, well below the 48dB or so you should expect.

At 10kHz the gain is hardly down at all, it should be only 20% of that at 1kHz.

These data indicate an error in the RIAA equalization. I'm sorry but it will be a day or two before I can analyze it, hopefully either that's enough clue, or someone else will have the critical insight before then!

[grufti]

In this case the scope worked just fine to document that the gain from this Eros is not what it should be. When the difference between the expected result and the actual is as great as 24dB then reading peak-to-peak voltages from a scope will do the job. Quite a few RMS reading meters can't be trusted at 20kHz and just about any scope will still display 20kHz accurately ... trade-offs everywhere.

[Paul Joppa]

Sorry, no useful insight yet, beyond my previous observations. The RIAA equalization is not working correctly - there's too little bass boost, and hardly any treble cut - and the gain is more than 20dB too low at 1kHz. I see two further useful things to do:

* Experimentally, you can look with the scope at various intermediate points in the circuit. I'd suggest the EF86 grid (T19/T27), EF86 plate (A6/B6 or T4/T12), 6922 plate (T9/T10), and 6922 cathode (C3/C8). Use the scope AC coupled while you have 20mV peak to peak into the input. Your previous frequencies are fine for this.

* Mechanically, inspect the 0.030 and 0.010 capacitors to make sure they are the right values and installed in the right places, and measure the 10.5K resistor.

The resistances you measured earlier are consistent with the circuit diagram, while the ones in the manual are not. Being away from my own Eros, I can't confirm either set yet, but I thought I should mention that. Those resistances indicate to me that the resistors in the RIAA, and the plate and cathode resistors, are all the correct value, hence my emphasis on the capacitors. Your DC voltages indicate the correct DC operating points.

A couple faint possibilities also occurred to me: (These are faint because I imagine you have inspected this things way too many times already - but I list them because they are possible causes of the observed symptoms.)

* Check the integrity of the grid and plate stoppers, attached to A9, B9, C2, and C7 (220 ohm) and C1 and C6 (100 ohm)

* Check correct attachment and good solder joints and correct orientation of the 100uF/160v electrolytic capacitors - I can see the possibility that without this capacitor, negative feedback to the screen grid of the EF86 would reduce gain. See p.28 of the manual.

This one is a real puzzler! At least we now know for sure that it is inside the Eros.