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Old 10th July 2007, 05:30 PM   #1
gy21 is offline gy21  Netherlands
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Question would this be a good preamp?

Click the image to open in full size.

Would this circuit be a good preamp on it's own? Any comments?
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Old 10th July 2007, 05:50 PM   #2
paulb is offline paulb  Canada
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My comment is:
I can't see the picture.
Oh, there it is. I had to enable picture-showing, I guess.

What type are the op-amps?
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Old 10th July 2007, 06:01 PM   #3
gy21 is offline gy21  Netherlands
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Click the image to open in full size.

here is a 2nd link to the picture. Opamps can be any good quality dual audio opamps. I currently use ad8620's.
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Old 11th July 2007, 07:04 AM   #4
gy21 is offline gy21  Netherlands
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anyone?
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Old 11th July 2007, 12:34 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
is vr201 a balance control?
get rid of it.
what is cap 261 doing. I would be tempted to short it out.
change r225 & r226 to >470k
I think c263 (100nF?) and R317 (1M0) are some kindof loudness function get rid of them.
Bypass each of the DC blocking caps with 1uF PP or PES caps.
Check your 8620 datasheet and ensure the chip decoupling on the PCB meets AD's minimum standard.
Is 8620 unity gain stable?
Is Vr202 is the main volume control?
It is buffered before and after. I would simplify this and get rid of the input buffer.

That leaves a pair of unity gain opamps and a pair of volume pots.
Nothing else.
That gets closer to what I said way back scrap the innards and use the case and PSU for a good preamp. Those discrete stages may be better than 8620.
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Old 11th July 2007, 01:10 PM   #6
Symon is offline Symon  United Kingdom
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The volume control looks like the active control similar to that used in D. Self's Precision Preamp 1996. And as such I think it works best fed from a defined low impedance. So I'd keep the input buffer.

I'm not sure but I think the 1M resistor is a protection against the wiper of the volume pot becoming detached. If you remove it you could get a large DC offset if the wiper looses contact.

I think the 100pF is there to ensure stability since the feedback ration of this stage varies with the setting of the volume control.
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Old 11th July 2007, 01:12 PM   #7
gy21 is offline gy21  Netherlands
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Quote:
Originally posted by AndrewT
Hi,
is vr201 a balance control?
get rid of it.
what is cap 261 doing. I would be tempted to short it out.
change r225 & r226 to >470k
I think c263 (100nF?) and R317 (1M0) are some kindof loudness function get rid of them.
Bypass each of the DC blocking caps with 1uF PP or PES caps.
Check your 8620 datasheet and ensure the chip decoupling on the PCB meets AD's minimum standard.
Is 8620 unity gain stable?
Is Vr202 is the main volume control?
It is buffered before and after. I would simplify this and get rid of the input buffer.

That leaves a pair of unity gain opamps and a pair of volume pots.
Nothing else.
That gets closer to what I said way back scrap the innards and use the case and PSU for a good preamp. Those discrete stages may be better than 8620.
Thanks AndrewT! Great Help.

Vr201 is indeed a sort of balance control.

Instead of cap c261 shorting i shorted out cap c217 and removed r227 (both channels)

bypassing blocking caps with 1uf film caps already done.

vr202-1/2 is the main volume control.

On the buffer thing, wich one should be removed then ic207 or ic208?

I checked the datasheet for the ad8620, i cannot yet find ad minimum chip decoupling required. I will search this.

I'm not sure with "Those discrete stages may be better than 8620" , are you talking about external discrete stages or something already on the board.
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Old 12th July 2007, 07:18 AM   #8
gy21 is offline gy21  Netherlands
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Last night I removed vr201 and replaced it by a 2k2 resistor.
R225 & R226 are changed to 560 kOhm (closest value to 470k i had)

Unfortunately when measuring voltage on the opamps (pin 8) I fried my AD8620's, don't exactly know what happened. But when measuring the relays stated rambling and I smelt burning plastic. After having a quick look I saw the ad8620's had a brown tan.

I soldered the soic ad8620 on a dip package with some resistor leads, maybe when measuring i shorted a pin or something.

Also the voltage on pin 8 is 18V, the ad8620 are recommended for 13V, some maybe they were running to hot anyway? I'm now back to the AD823 wich can run on 18V.

How can the supply voltage be lowered somewhat?
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Old 12th July 2007, 08:45 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
your original schematic showed a few discrete stages. These could probably be used IF the RIAA filtering is removed from them.
You will need to check that each of them are unity gain stable.
They are probably already working on the 18Vdc supplies you have.

Why choose a 12V chip when your supplies are 18V?
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Old 12th July 2007, 09:52 AM   #10
gy21 is offline gy21  Netherlands
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Quote:
Originally posted by AndrewT
Hi,
your original schematic showed a few discrete stages. These could probably be used IF the RIAA filtering is removed from them.
You will need to check that each of them are unity gain stable.
They are probably already working on the 18Vdc supplies you have.

Why choose a 12V chip when your supplies are 18V?
i need those discrete stages for my turntable, so that's not really an option.

yea why choose a lower voltage chip, your right that was not a good idea. Is it easy lo lower the supply voltage for the opamps?
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