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Old 25th October 2008, 12:34 AM   #1
gain is offline gain  United States
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Default input stage and VAS testing

Hi,

was just wondering if i could use this circuit to listen to the VAS and input stage only without the OPS connected for testing/experimentation purposes.

thanks.
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Old 25th October 2008, 04:30 AM   #2
gain is offline gain  United States
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ok, y'alls being all quiet, whats up with that?

- have you ever wanted to listen to/scope out your input stage + vas before connecting to OPS? sure you have imo.

- have you ever repaired an audio amp with blown channel and wanted to make sure VAS / diver stage ok before installing new outputs??? sure you have imo.

- i don't expect a PhD analysis on the circuit i posted, complete with maths. just a common sense opinion, plain language description.

i know you are are more than capable of understanding and explaining whats going on here, and that is what is frustrating to me why everyone is so quiet.

speak up!
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Old 25th October 2008, 04:49 AM   #3
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I would add a pair of drivers and listen to it / test it. It doesn't need a full OPS. Pick the NFB off the drivers output.
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Old 25th October 2008, 05:01 AM   #4
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yeah, I'm guilty of runnin' amps without OPS. I do it for 2
reasons, sometimes I just don't have the funds to afford the OPS
and I'm cheap and dont wanna see 20$ of outputs go up in
smoke.

I do it a little different than above. First I'll hook the drivers
up with 2 47 ohm resistors, then run headphones off of a
220 ohm resistor or directly run a cheap 16 ohm speaker.

Since the drivers just "follow"or buffer the VAS one can check
voltages,hot components, and bias (you will be grossly
underbiased when you do hook up OPS,which is good).

With this I DO NOT burn outputs (I always drool over all the
burnt OPS devices I see in the quasi threads)
and get to test amps out before going high power/ current.
OS
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Old 25th October 2008, 05:08 AM   #5
gain is offline gain  United States
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Hi MJL,

thank you for responding to me several times in various threads. i am professionally and personally vary grateful.

if you have time in your busy day, i am still mostly beginner with diy audio so here are a few basic questions:

- by driver you mean the first (inner) darlington transistor (assuming EF topology)? then hook the 1K trafo between emitters of top and bottom drivers? possibly with emitter resistors?

- must the NFB loop come off the divers? may it come off the VAS leg instead, especially if NFB tap point is trimmed to ~0V?


MJL, your avitar rules! so does mine just yours is more of a closeup.
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Old 25th October 2008, 05:23 AM   #6
gain is offline gain  United States
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Quote:
Originally posted by ostripper
I'm guilty of runnin' amps without OPS. I do it for 2
reasons, sometimes I just don't have the funds to afford the OPS
and I'm cheap and dont wanna see 20$ of outputs go up in
smoke.

yeah me too buddy!


Quote:
Originally posted by ostripper
i a little different than above. First I'll hook the drivers
up with 2 47 ohm resistors, then run headphones off of a
220 ohm resistor or directly run a cheap 16 ohm speaker.

i like. were headphones in series or || with drivers?


Quote:
Originally posted by ostripper
iSince the drivers just "follow"or buffer the VAS one can check
voltages,hot components, and bias (you will be grossly
underbiased when you do hook up OPS,which is good).

With this I DO NOT burn outputs (I always drool over all the
burnt OPS devices I see in the quasi threads)
and get to test amps out before going high power/ current.
OS

me too. i crank down the bias trimmer to 0V between base and emitter of Qbias. Let her run in class C for a little till you make sure everything is working ok. then spread the bias out to class B or AB until crossover distortion is acceptable to your (or optionally, your friends) ears.

thanks for the post.l
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Old 25th October 2008, 05:47 AM   #7
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Quote:
were headphones in series or || with drivers?
Just hooked in series with output through 220 ohm resistor.
Once , since these amps can put out 60V p-p , I nearly
blew my ears out and fried headphones doing this.

Now I just use a cheap junkbox radio speaker to do this.

One can get a idea of Xover distortion/bias using test tone
as you get at /or below 1.2V across VBE.
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Old 25th October 2008, 07:09 AM   #8
gain is offline gain  United States
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Quote:
Originally posted by ostripper


Just hooked in series with output through 220 ohm resistor.

cool. just like regular headphone outs.


Quote:
Originally posted by ostripper

Once , since these amps can put out 60V p-p , I nearly
blew my ears out and fried headphones doing this.

Now I just use a cheap junkbox radio speaker to do this.

i can relate. don't know how many times i almost blew my eardrums out from trying something like this. note to 18 yo self: use a vol control pot!!!


can anyone recommend a reliable way to order parts for my turntable?
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Old 25th October 2008, 11:49 AM   #9
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Quote:
Originally posted by gain

- by driver you mean the first (inner) darlington transistor (assuming EF topology)? then hook the 1K trafo between emitters of top and bottom drivers? possibly with emitter resistors?

Hi,
I'm far from an expert, just learning here myself.

Basically yes, construct your driver stage as it would be in the final amp (see below). I don't think you need the transformer. For impedance matching, put an appropriate value resistor in series with the output.
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Old 25th October 2008, 09:27 PM   #10
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Default Re: input stage and VAS testing

Quote:
Originally posted by gain
Hi,

was just wondering if i could use this circuit to listen to the VAS and input stage only without the OPS connected for testing/experimentation purposes.

thanks.

I always test any driver board I build by applying the VAS output back into the -ve LTP.
I like to know this is correct before even considering connecting output transistors.

The scope quickly shows up any problems without blowing a set of output transistors.
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