Eros Phono finished! Plus question about gain

[dave-tx]

Completed the Eros this afternoon.  All resistance and voltage tests passed (within tolerance).  Plugged it in, and was immediately blown away by the sound quality.  Huge upgrade over my existing ARC SP-9 phono stage (which I actually liked a lot, prior to today).  (Will post a photo in the Gallery section later)

My only question would be about gain.  Both the Eros and the phono stage of the SP-9 exhibit the same "shhhh" sound at very high volumes when no music is played.  The Eros (connected to the "spare" input on the SP-9), however, needs the volume turned dramatically higher to reach the same levels of volume when listening to music.  Measured casually on the line-out into a recording device, my Eros is about 20+ dB lower than the SP-9 phono stage. 

I'm running this with a Rega RP-3 and Rega Exact cartridge.

Is there a mod or adjustment I can do to increase gain?

Thanks,
David

 

[Grainger49]

The gain of the Eros is fixed.  Increasing it would make the noise louder.  But give the tubes about 100 hours to run in.  Often the noise goes away.  My Eros is dead quiet at the levels I listen to, mid 80dB range, between tracks.

 

[galyons]

Something doesn't sound right to me.  The SP9 is not a tube based preamp. It is a hybrid and most of the  gain comes from n-fets and mosfets.  The Eros is all tube. The SP9 has 47dB gain phono to line. The line is 21dB, so the total gain is 68dB.  The Eros is 50db.  So the total gain, Eros to SP9 output through the line stage, is 70dB. That is low output moving coil range. .3-.4mV. 

Your  MM cartridge is very high output at  6.8 - 7.2mV.  It does not need anywhere near the gain of the Eros plus the SP9. More like half, 36-40dB is more than enough.  If everything is functioning properly on the Eros, you do not need more gain, you need far less.  IIRC correctly, the SP9 has a jumper to reduce the line stage gain to 14dB.  What is the sensitivity of your amp(s)?

If you are hearing the same "shhhh" sound from the SP9 phono and the Eros, I would look elsewhere for the source.

Sorry to sound negative.  Had an SP9 for a short while, (not short enough), so please read with my bias in mind!!

Cheers,
Geary

 

[galyons]

Just a thought, also make sure that you have not connected the Eros into the SP9 tape loop.  That bypasses the line stage, hence, 20dB less!!

Cheers,
Geary

 

[dave-tx]

Thanks for the tips; I've double-checked all of the connections to my preamp, and everything is as it should be.

Bear with me a bit, I haven't done much in the way of gain calculations (or even given it much thought, to be honest) for the entire signal chain before.  My first thought was what you were saying - this cartridge should be giving me a LOT of output to work with...which is why my first impression was that the gain out of the Eros might be low, but as Grainger's post reminded me, that would imply that the noise floor would also be lower. 

My power amp is a Bryston 3B-ST, sensitivity 1V for 120W into 8ohms, unbalanced.

I'm thinking of pulling out the o'scope and running a signal generator through the Eros to measure the actual gain I'm getting...unless that's a bad idea?

 

[galyons]

My power amp is a Bryston 3B-ST, sensitivity 1V for 120W into 8ohms, unbalanced.

If we use 7mV for your cartridge, you need 143 gain, (1/.007),  to drive the Bryston amp, 1V,  to full power.  That is 43dB gain, everything blasting at max volume.  So, something just doesn't add up.  You're positive that the Eros is not in the tape loop?

Cheers,
Geary

 

[Grainger49]

Dave,

I have to ask, what do you mean by, "at high volumes with no music playing?"  If you run the volume wide open with no music playing this is not a test that has meaning.  It is brought to us by neurotic audio reviewers.  But if you mean when listening at high volume between tracks that is more meaningful. 

So if it is the latter let the tubes run in as I suggested above.

 

[dave-tx]

My power amp is a Bryston 3B-ST, sensitivity 1V for 120W into 8ohms, unbalanced.

If we use 7mV for your cartridge, you need 143 gain, (1/.007),  to drive the Bryston amp, 1V,  to full power.  That is 43dB gain, everything blasting at max volume.  So, something just doesn't add up.  You're positive that the Eros is not in the tape loop?

Cheers,
Geary

I'll check again this evening about the tape loop, what you're saying seems to be the only logical conclusion. 

The most curious thing, to me, is looking at the levels on my recording device; normalized to where the internal SP-9 phono stage peaks at about 0dB on the recorder's meter, the Eros isn't even reaching -20dB.  Monitoring the tape output should remove the line gain, and I would expect the Eros to be a few dB higher than the internal phono when measured this way.  Does that seem to be the correct conclusion?

Thanks for the help,
David

 

[dave-tx]

Dave,

I have to ask, what do you mean by, "at high volumes with no music playing?"  If you run the volume wide open with no music playing this is not a test that has meaning.  It is brought to us by neurotic audio reviewers.  But if you mean when listening at high volume between tracks that is more meaningful. 

So if it is the latter let the tubes run in as I suggested above.

Ah, I see what you mean; My statement was a little of both.  Volume wide open with no music playing had the noise where I expected - slightly less hiss than the same wide open test of the SP-9 phono stage with no music playing.

Also, listening at what I would consider normal listening levels (not particularly high volume), between tracks.  I have to crank the preamp gain up significantly higher when listening to the Eros than I do with the SP-9 phono stage to get the same level of music volume.

Thanks for the help,
David

 

[Paul Birkeland]

You will get roughly 1.5V of signal out of the Eros with 5mV in.

The AR SP-9 gives the following specs:
"Line-stage gain: 21dB to main out (CD stage 12dB lower gain), 0dB to tape out."

If we apply the full 21dB to the signal from the Eros, that will give you 16V at the output, and of course this will drop a lot if you're using the tape input or CD input.

It looks like the actual gain of the phono stage in the SP-9 is ~46dB, which is similar to the Eros, but it goes through all the gain circuitry in the SP-9, including the preamp circuit, hence the total of 67dB.

It is also very possible that all of the inputs are padded on your particular preamp, I would recommend downloading a 60Hz test tone to your phone, then sending a very low level 60Hz tone through your preamp at max volume.  Keep the input level down around .1V AC (measure with your volt meter), then measure the output.  You can do this for all the inputs except the phono input, as that circuitry has an EQ applied that will give you erroneous readings.  I would recommend doing this regardless of any other conditions, just to understand how your preamp may be optimized.

Ultimately, two different phono preamps from two different manufacturers are very unlikely to have the exact same gain, so having to adjust the level between the two is not surprising.
-PB

 

[dave-tx]

I did a little bit of testing of the preamp inputs and at quick glance found that they behaved as I would have expected; I will look more closely at them this weekend when I have time.

Although I'm still completely mystified about the lower output in general, I'm beginning to agree with Grainger that I just need to let the tubes run in and re-evaluate after that.  This may be just a case of my paranoia in building my first kit, thinking there must be something I need to adjust. 

I do appreciate all of the helpful suggestions!
-David

 

[Grainger49]

I don't think you are going to pick up any gain.  But you will see lower noise.  Dan uses my Eros "Mini-Review" from a weekend with the Eros on the product page.  My longer review is here after I bought and built my own:

http://www.bottlehead.com/smf/index.php/topic,2258.msg17408.html#msg17408

 

[grufti]

I am currently away from home and can't do this myself.

If somebody could generate 1-2 minute long a 1kHz tone [0 dB point of the RIAA curve] that plays back at 5 mV through a standard consumer DAC [which is supposed to output 2 V max.] then it would be relatively easy to get a good first idea of the gain at the output of the Eros.

Computers with a soundcard seem to be more common that function generators.

 

[Paul Birkeland]

I am currently away from home and can't do this myself.

If somebody could generate 1-2 minute long a 1kHz tone [0 dB point of the RIAA curve] that plays back at 5 mV through a standard consumer DAC [which is supposed to output 2 V max.] then it would be relatively easy to get a good first idea of the gain at the output of the Eros.

Computers with a soundcard seem to be more common that function generators.

Considering that digital playback mechanisms do not have regulated AC outputs, it is impossible to generate a 5mV tone for someone else.

You can, however, download a function generator to your phone, set the frequency to 60Hz, dial down the volume while measuring the output with your meter till it hits 5mV, then turn the frequency up to 1K.

-PB

 

[dave-tx]

After a couple weeks of poking and prodding, I'm still of the opinion that something's not quite right with my build.  Re-measured all of the resistances and voltages, thought I'd post the ones that are off and see if any of them are critically out of range.  Any opinions?

Resistances
Terminal Strip 2 and 14: 85.5k (manual lists 71k)
TS4/12: 74.8k (60k)
TS9/10: 46.6k (39k)
Socket A6/B6: 74.8k (60k)
Socket C1/C6: 46.7k (39k)
Socket C2/C7: 75k (60k)

Voltages
Kreg (1.0V) A:1.60V, B:1.58V
OB (100V) A:95.8V, B:94.8V
bA (0V) A/B: 0V
OA (170V) A:170.2V, B:166.9V
IA (225V) A:224.5V, B:220.9V
breg (100V) A:97.1V, B:95.8V

The voltages seem to be within reason, but there are a few resistances that seem significantly off.  Inspection hasn't revealed any poor solder joints, but maybe I need to reflow some to make sure.

Thanks,
David

 

[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