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Old 3rd July 2002, 11:25 AM   #106
Freddie is offline Freddie  Sweden
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ehh?

I'm not completly sure what you mean

If I increase the value of R6 to 1k1. I will get 0,75V across Q4.

/Freddie
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Old 3rd July 2002, 11:26 AM   #107
Duo is offline Duo  Canada
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also, it would be a good idea to add a current mirror to your long tailed pair, it'll achieve proper balance upon application of power and improve distortion and linearity figures... To see the mirror, just peek at my diagram on this post, you'll see it... It's quite easy to implement and I don't believe it requires any modification of the curcuit mathwise or currentwise...
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Old 3rd July 2002, 11:29 AM   #108
Duo is offline Duo  Canada
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okay, sorry for my bad description of the current thing...

you must reduce the value of R7 to compensate for the voltage drop accross the 47 ohm resistors

essentially, the current through the curcuit will stay the same, but the voltage drop accross R7 will be less and the voltage accross R6 will stay the same...

say, for example you have 2mA flowing down your LTP, thus R7 drops 1.12 volts and that's that, but say you add the 47 ohm resistors, and you want .5 volts accross them, then R7 must only drop only .62 volts and thus be reduced to 310 ohms respectively to remain at the same current. but this is only an example since .62 volts is much too little accross R7... You'd basically want between 3 and 4 mA flowing down so each side of the LTP gets between 1.5 and 2 mA to idle at and there's enough voltage drop accross R7 to keep the current sink transistor happy at all times... i hope this explains a little better
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Old 3rd July 2002, 11:38 AM   #109
sonnya is offline sonnya  Denmark
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There are some parameters you want to have right first.

To minimize DC-offset you should set the idle current to a optimum level.

So in the VAS stage you want a idlecurrent ~9.5mA. At this current the voltage drop across R9, R10 would be ~51V(R9 and R10 should be .5W resistors).
=> R40 * 9.5mA = ~.95V across R40.
=> .95V+.65V = 1.6V
Idle current in Q1 => 1.6V/R6 => 1.6V/560R = ~2.86mA.

At this point your current through Q3 should be 2*2.86mA => R7 = ~250R.

This is not an optimum value for MPSA06. Use 2N5550/5551 instead.
With MPSA06 the optimum idlecurrent would be around 30mA where the HFE curve is flat.
With 2N5550/5551 2.86mA is perfect. It is faster and the Cob (the capacitance between C-B) is lower than with MPSA06.

Sonny
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Old 3rd July 2002, 11:42 AM   #110
Freddie is offline Freddie  Sweden
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Sonny,

Thanks a lot for the detalied reply.

/Freddie
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Old 3rd July 2002, 12:04 PM   #111
Duo is offline Duo  Canada
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shoot, I was thinking that you wanted to compensate for addition of tail resistors, i must be tired, but what sonny just described is, of course, correct, so never mind my babble there. But it would still be an interesting idea to add a current mirror...
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Old 3rd July 2002, 12:07 PM   #112
sonnya is offline sonnya  Denmark
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It's okay to babble!!!

Current mirror is a nice thing to add... Some like it .. some don't .. I do like them.

Sonny
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Old 3rd July 2002, 12:10 PM   #113
Duo is offline Duo  Canada
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Well, I know it's okay to babble, but I hate it when I get real into detail without first thinking clearly what I was getting into detail about... Anyhow, yes, a current source is a nice addition, but not a requirement, kind of like an opinion basically
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Old 3rd July 2002, 12:12 PM   #114
Freddie is offline Freddie  Sweden
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Default Final version?

Here it is once again.

BTW: Is the value of R11 and R12 critical.

Thanks again Sonny.

Duo, I won't add a current mirror. Lower distorsion does not always mean better sound, and I don't want to make this amp more complicated either. For example I like the sound of my BoZ preamp better, than my OPA604 based preamp. Guess which one has got the lowest distorsion

/Freddie
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Old 3rd July 2002, 12:17 PM   #115
Duo is offline Duo  Canada
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Yeah, like I said, just an opinion, if you're trying to keep it simple, I agree, it's a good way to go about things for sure

Also, I believe I read on Rod Elliot's pages that R11 and 12 arent
very critical, but let sonny make a proper point of that cause he probably has more experience with compound pairs

And i'd also have to agree that lower distortion doesn't always have to do with better sound. I 've seen some pretty high distortion numbers that meant nothing when you listened to the amp
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Old 3rd July 2002, 12:25 PM   #116
Duo is offline Duo  Canada
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also, thanks to info from sonny, I'll change my input trannies to 2n5550 and 5551, the've got perfect specs
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Old 3rd July 2002, 12:33 PM   #117
Duo is offline Duo  Canada
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hmmm, i was just thinking, i didn't work out my resistors properly, can you give me some pointers for biasing the vas in here sonny, it would be much appreciated!
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Old 3rd July 2002, 01:04 PM   #118
sonnya is offline sonnya  Denmark
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Quote:
Originally posted by Duo
also, thanks to info from sonny, I'll change my input trannies to 2n5550 and 5551, the've got perfect specs

uhhmmmm!!! 2N5550/5551 is npn, The complementary pair is 2n5401 - PNP.

First find the datasheet on the fairchild BJT's in the VAS stage. Then i will get back later.

Freddie and Duo : R11 and R12 will form a pole with Cob in the output.
Also : Try figure out wich idle current you will have in the drivers and output in the compound outputstage.

Sonny
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Old 3rd July 2002, 02:14 PM   #119
Duo is offline Duo  Canada
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okay, well, the current in my OT's is variable, but I'm having trouble with the current mirror... have I got the VAS connected properly?? It doesn't seem like it'll bias properly will it?
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Old 3rd July 2002, 02:41 PM   #120
sonnya is offline sonnya  Denmark
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The current through the driver pairs in the compound outputstage will not change that much. Only a few mA's.

Replace R21 and R20 with 100R instead of 27R. There can easely be 20mV difference in the VBE of to identical BJT's With 100R a mismatch of 20mV will give a mismatch of 200uA. With 27R this will be 1mA!

KSD401 should at least be HFE class Y not to load your current mirror to hard.
Have you checked the pin combination on the new BJT's?.
Are you sim or building it?

You should have something around 390mV across R20 and R21 @100R.??

Sonny
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