gyraf stereo pultec balancing.

[cwatkins]

I've built a stereo pultec, and I have one side
that is 2db off from the other, I have verified
that without the front panel connected, the in
and out looped up that it's still short the two db..

Most of the componet's are exactly the same,
except the 4700u cap on the power board..

Is it realistic to expect them to be closer in volume,
I tried a different set of tubes and checked most of the
resistor values, where would one look for the gain differences?

It's just that if you put this in a mastering chain, it's kind of a pain
to adjust / readjust a A/D for the difference and then it's also slightly
different when the eq is engaged, as opposed to in bypass.

Still quite happy it isn't overheating, but curious now how to get it
spot on.

 

[CJ]

You could tweak the plate resistors.

Do you have stepped controls? If not, kind of a waste to tweak it , since the pots will have a 10 percent tolerance.

 

[rafafredd]

Yes, if you are using pots before the make up amp, the unbalance between the two pots might be causing it... There are pots up to 20% tolerance...

 

[cwatkins]

Which are the plate resistors?

Well, it's 2db off in bypass mode. and also when the out is connected
to the in on the power board, so my assumption there is whatever
I do should be done on the power board?

The thing is, I felt I needed to be able to start off passing audio through it with a equal balance, long before I worried about the eq kicked in. I mean, I suppose if you consider kicked in it will be off so whats the difference anyhow, I guess that's an argument, but it still seems to be that 2db weeker and then you find yourself mal-adjusting the eq balance on the other side.

 

[alk509]

[quote author="cwatkins"]Most of the componet's are exactly the same[/quote]

Most? Which ones aren't (besides the power supply cap)?

By "exactly the same" do you mean just the same part number, or did you actually measure and match each component? Remember resistors and caps have a tolerance spec...

Did you use matched tubes?

Peace,
Al.

 

[chrissugar]

In my mastering version of Pultec I used 1% resistors and selected identical pairs from a large number of resistors. Also made pairs of capacitors and inductors. I used 24 step rotary switches. The difference between the two channels is maximum 0.02dB at any freq and level position. With pots you have 10% tolerance.

I sugest you to remove the filter section first and tweak the gain stage untill you have two identical channels.

chrissugar

 

[gyraf]

Gain of the SRPP stage can depend somewhat on the mu of the used tube - because we're running no-feedback amplification. Try swapping the tubes to see if gain follows tube. If so, then either trim input level a bit down on the "high" side - or select tubes that gives the same gain..

Jakob E.

 

[chrissugar]

Yes, Jakob is right, I forgot to say that I had to swap some tubes to match the gain for the two channels.

chrissugar

 

[cwatkins]

component part #'s are the same. accept the one cap. which just has a higher voltage rating.

I thought, before I purchased several different tubes that the gain changed
with the tube. What I didn't try was mixing tube brands now though. Maybe I can fix the gain by swapping types.

It also seemed to get a hair better after being on for awhile, so I was
thinking that was the tubes too.

I'll try mismatching..

 

[alk509]

[quote author="cwatkins"]component part #'s are the same.[/quote]

Again, are the components matched? It's not enough to just use the same part numbers: ELECTRONIC COMPONENTS HAVE A TOLERANCE SPEC!

[quote author="cwatkins"]I'll try mismatching..[/quote]

Don't. You might get the same small-signal gain on both sides, but the sound will be different.

Match all resistors, pots and caps, and get matched tubes of the same brand for both channels.

Peace,
Al.

 

[alk509]

It just dawned on me that maybe you don't know what I mean by matching... So here we go:

Components have a tolerance spec, usually given as a percentage of the individual part's value. So a 100K, 10% resistor can have a value of anywhere from 90K to 110K (100K +/-10%, get it?). The same applies for caps, pots, inductors, etc.

If you switch tubes in both channels and the same channel is always 2dB lower than the other, then chances are the difference is not due to the tubes you're using, but to the other components around them.

In the GPEQ, I'd check and match the resistors and pots common in both "bypass" and "EQ" modes, as well as the ones around the SRPP's. I'd match the caps as well, but that's just me. If you have a capacitance meter and a bunch of caps, you might as well.

And use matched tubes. Most tube shops will match tubes for you for couple of bucks.

Sorry if this is all obvious stuff that you already know!

Peace,
Al.

 

[gyraf]

Most tube shops will match tubes for you for couple of bucks

Yes. But usually not for mu. I think the variation of tube gain can be more than what comes from standard components. Look at the tubes first, then the other stuff.

And btw: let the tubes heat for at least an hour before you try to judge parameters related to gain. Specially with factory-new tubes.

Jakob E.

 

[alk509]

[quote author="gyraf"]I think the variation of tube gain can be more than what comes from standard components. Look at the tubes first, then the other stuff.[/quote]

Nah-ah... If he keeps swapping tubes, and the same channel is always 2dB lower, then chances are it's not the tubes...

Peace,
Al.

 

[gyraf]

Right you are, sir.

I wasn't paying attention..

Jakob E.

 

[cwatkins]

I'm pretty sure it ended up being the same channel, I'll go back and look.

As far as component matching ok, I know about 1% vs. 10%.

Everything is chuckd's parts, except for the transformer fix.

Which, guess I *didn't* mention this, I am only using one set of

transformers to power both now, instead of a total of 4.


So to recap:

I have isolated it to the power board by:

Connecting the out to in on the "srpp"? board. on both boards, one is 2db
louder than the other.

It is of course the same in bypass.

From memory: I swapped tubes and thought it moved, but I must have been mistaked because I have now been through 4 more tubes, (2 new sets) (I like to shock myself and drop the tubes, I don't discharge caps well.)

I have tested specificly, each resistor, and paid special attention to
the input 10k and output 2k2, all within a couple ohm's of each other.

(I do not have the input transformers... have considered trying this.)

The only different component is the 4700uf cap, because the max voltage on it's different (picked it up local), I can look for the two original ones I used, originally toasted the bridge rectifiers and suspected that cap on one of the boards when I was testing the transfromers. (this is on the board btw, that appears to be at the higher,correct volume, the one that takes the transformer power first) So there are a couple traces there that are wired by hand.

Because of that little incident there is a new pcb on the way, because I'm a little leary of that causing a gain issue, but then again, I'm a newbie,
so I'm just siding on caution there..



Hmmm...

You know... I didn't wait an hour though... And I might want to wait for that other side to heat up with a different brand..

 

[rafafredd]

What transformers are you using for input/output? Maybe it´s a ratio problem... Sometimes transformer have a high tolerance. You must match those also.

 

[digichild]

[quote author="chrissugar"]In my mastering version of Pultec I used 1% resistors and selected identical pairs from a large number of resistors. Also made pairs of capacitors and inductors. I used 24 step rotary switches. The difference between the two channels is maximum 0.02dB at any freq and level position. With pots you have 10% tolerance.

I sugest you to remove the filter section first and tweak the gain stage untill you have two identical channels.

chrissugar[/quote]

where did you get this switches from?

 

[chrissugar]

"where did you get this switches from?"

I use ELMA 04 series 24 steps. Bought them from a local dealer.

chrissugar

 

[miszt]

In my mastering version of Pultec I used 1% resistors and selected identical pairs from a large number of resistors. Also made pairs of capacitors and inductors. I used 24 step rotary switches. The difference between the two channels is maximum 0.02dB at any freq and level position. With pots you have 10% tolerance.

I sugest you to remove the filter section first and tweak the gain stage untill you have two identical channels.

chrissugar

what tolerance are your inductors matched to?

do you know how much of a difference it makes using 10% as opposed to 1%? if its just 1hz/0.01db or 1khz/10db, or?

 

[gyraf]

..11years later.. 

To sum up on original question, the attenuation through the pultec filter depends on the ratio of Hi-boost (10K) to High-cut (1K) potentiometers. And as most potentiometers are 10-20% tolerance, there's plenty of room for a mismatch.

For matching channels, a trimmer pot can be put in series with the clockwise end of the pot, allowing for true 1:10 ratio, and thus filter attenuation.

Jakob E.