EQN build thread

[dandeurloo]

Just tested my eq's.  One of them is perfect and sounds amazing! Seriously amazing sounding eq!

The other one has 5db less output.  I went back through and double check all the resistors and everything is right.  I put the other boards op amp card in to this eq and it was the same so its not the op amp cards.  Any suggestions on how to locate the problem?

Thanks

 

[zayance]

RV4 is to adjust level difference of 'eq in' vs 'eq out'. adjust RV4 with all the frequencies set to "off" to get the same level at both positions of the 'eq in' switch.

Tried this?

 

[dandeurloo]

yeah, it doesn't have enough range to make up the 5 db difference. 

I got the other eq with in .5db so that one is good.  And it sounds really really good!

 

[zayance]

Quote from: Marcocet on June 30, 2010, 08:41:45 pm
I have noticed that there's about a 1db gain loss when they're inline, both while engaged and bypassed

Have you tried adjusting RV4 (fader) to make up the loss?
If you have, then R29 needs to be reduced to increase the gain in the output amp.

 

[dandeurloo]

right.  

The thing is, I have 2 eq's built with the same parts.  One is working properly with a .5 db difference with the eq in vs. out.  

The second eq is 5 db difference.  I have a 750R in r29 of both eq's.  All 1% resistors except the 1/2 and 2 watt 12r.  Shouldn't they be a lot closer?  I could change r29 in the second one to a lower value like 600r.  But I would think they would be closer in general.  Makes me think something might be off some where.

If I did change r29 in the second eq would it on change the output level of the eq when it is "in" only.  If so that may be the solution if I can't seem to track down why it wouldn't be closer to the first eq. 


Thanks

 

[peter purpose]

No... It's not a matching question, but an inadvertant mistake.... sleep on it.

 

[dandeurloo]

Ok, I glad you have a hunch!  I'll keep poking at it and see if I can find it.

I'm trying to understand the schematic a little better.  Am I correct thinking that the green line is the path if the eq is not switched in?  And the path with out any of the bands switched in but the "EQ IN" goes through R52 (10k) on the schematic?

I'm trying to find what is in the signal path once the EQ switch is IN.

Thanks

 

[briomusic]

In the schematic all the colored lines are voltages, green being 0Volt. The audio path is the black line coming out of the bottom of the input transformer. Tracing this will eventually lead you to the eq in/out switch

 

[peter purpose]

No... It's not a matching question, but an inadvertant mistake.... sleep on it.

I was very drunk..... and still have no answer.

 

[briomusic]

my guess is transformer wiring (for the 5dB loss)

 

[dandeurloo]

i've checked the tranny's about 8 times thinking the same thing.  It is correct.  I wonder if I should re flow the board? 

Could the difference be in the inductors? 

Any good test points?

 

[shabtek]

could the leads be wrong color?


also, is the hole too tight for Eds xfmr?

 

[dandeurloo]

In the schematic all the colored lines are voltages, green being 0Volt. The audio path is the black line coming out of the bottom of the input transformer. Tracing this will eventually lead you to the eq in/out switch

Ok, clearly a noob question.  The eq "in/out" switch is right after R73 on the schematic correct.  So the problem I should be after that switch since the level is fine when the unit is not engaged.

Its even a little more confusing for me since the shematic numbers don't match the boards but should be able to get it if I can start circling the area a little.

Thanks

 

[dandeurloo]

could the leads be wrong color?


also, is the hole too tight for Eds xfmr?

Well, I guess they could be but my other eq turned out fine.

The leads fit nicely.

Thanks for suggestion!

 

[briomusic]

dan, just to clarify, you get the 5dB level difference with:
1) eqn in the signal path, eq switch on 'out'
2) eqn in the signal path, eq switch on 'in', all frequencies set to off.
3) both

IIRC RV4 is active even in "soft bypass" which is what the eq in/out switch is. the signal still goes through all the transformers and the out-amp.

Can you toneprobe/scope a bit and find out if the level difference occurs at R7 or C48? (I guess you would have to check on both the good and the bad unit)

 

[dandeurloo]

dan, just to clarify, you get the 5dB level difference with:

2) eqn in the signal path, eq switch on 'in', all frequencies set to off.

Can you toneprobe/scope a bit and find out if the level difference occurs at R7 or C48? (I guess you would have to check on both the good and the bad unit)

The second one is what is happening.

Not sure how to toneprobe?  I'll goggle it and see.



Thanks

 

[zayance]

Not sure how to toneprobe?  I'll goggle it and see.

Toneprobing, means send a tone for example a sine wave at 1Khz, and use an oscilloscope to find that signal in the circuit with the "probes" of the Oscillo.

 

[briomusic]

If you don't have access to an oscilloscope you can just use any active speaker (I use a guitar practice amp). Connect the grounds together and put a 1uF (or similar value) polyester cap in series with the input to protect against accidentally hitting DC. now you can poke around your circuit and see what your signal is getting up to on its way. where it gets louder/quiter or lost altogether.

 

[haima]

** EDIT - oops, briomusic beat me to it  ;D

OR if you don't have a scope you can try using a little amp & speaker to follow the test tone though the circuit.

you'll need a capacitor in series with the probe to block DC. you can even make a probe out of axial capacitor, some heatshrink and some shielded cable, using the cap lead as the probe. just make sure the cap and the cable are rated for the power supply voltage you are working with - 63v is plenty for most solid state stuff, but you might need 500v+ for some valve/tube circuits. terminate the shield at the amp end and just cut it off an insulate it at the probe end.

you'll also need an alligator clip to tie the ground of the amp to the gear you are testing.

there's some good diagrams/pictures around on the net - google "audio probe" or debugging effects pedals etc...

http://www.diystompboxes.com/pedals/debug.html etc

it's a good tool to have, even if you have a scope too - sometimes your ears will tell you things you can't see, and vice versa.

 

[Holger]

Gents, my amp stage doesn't work. I am seeing and hearing my signal at the base pin of TR1. The EQ itself works as expected, only the gain stage doesn't.
I see 24V at the + pin of C53, but no voltage after that cap. I've exchanged C53, HS1 etc, still no supply voltage there. Any hints for me?