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Squeezing maximum gain from one dual op-amp?
Squeezing maximum gain from one dual op-amp?
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Old 7th February 2018, 05:54 PM   #21
scottjoplin is offline scottjoplin  Wales
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Using the triangular op-amp symbol for two amps confuses my simple mind
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Old 7th February 2018, 07:15 PM   #22
Mooly is offline Mooly  United Kingdom
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Squeezing maximum gain from one dual op-amp?
Maybe, but it is something you see a lot in service manuals. Although it is an opamp, its a little different to the generic types we use today. This one is a dedicated high gain device and self biased for single rail operation. It still follows the rules though.
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Old 7th February 2018, 08:11 PM   #23
rayma is offline rayma  United States
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Quote:
Originally Posted by Eps View Post
Somehow I'm only managing to send the circuit into oscillation in this format.
I don't see a minimum gain for stability spec, so let's assume x21 is ok.
If the nfb resistor to ground is 750R, then there should be a 7.5k
resistor between pins 6 and 7, instead of the short shown in your sketch.

Last edited by rayma; 7th February 2018 at 08:13 PM.
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Old 9th February 2018, 04:25 PM   #24
Eps is offline Eps  United States
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Originally Posted by Mooly View Post
You seem to be missing a resistor between pin 6 and pin 7.

The overall gain is set approximately by the ratio of the resistor between pin 6 and 7 and the resistor from pin 7 to ground (via its cap of course). No resistor gives unity gain (and it may not be stable like that).
Thanks, yup that was it!! So I seem to be getting a nice strong gain around 7-8k between pin 6/7 without an intolerable amount of distortion.

On another note, do you happen to know of any good references for an easy DC bias record circuit? For some reason I am only able to find AC bias schematics. I know DC bias is much lower quality but I think I need to stick with DC because I can only use a 9v DC wall adaptor for my purposes.
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Old 9th February 2018, 04:32 PM   #25
Eps is offline Eps  United States
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Quote:
Originally Posted by rayma View Post
I don't see a minimum gain for stability spec, so let's assume x21 is ok.
If the nfb resistor to ground is 750R, then there should be a 7.5k
resistor between pins 6 and 7, instead of the short shown in your sketch.
Haha that's pretty funny as I had also arrived at 7.5k by just listening and finding the highest gain without a ton of distortion. Thank you.

Since this circuit is really susceptible to ground hum and RF is it ok to use a 12v DC power adaptor that ties earth ground to neutral? I have done some tests with this circuit and it seems to have helped a good deal, not changed any of the pin voltage readings, and seems to not kill it.
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Old 9th February 2018, 04:34 PM   #26
rayma is offline rayma  United States
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Originally Posted by Eps View Post
Since this circuit is really susceptible to ground hum and RF is it ok to use a
12v DC power adaptor that ties earth ground to neutral?
Try it out, should be ok.

Last edited by rayma; 9th February 2018 at 04:40 PM.
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Old 13th February 2018, 03:16 PM   #27
Eps is offline Eps  United States
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Quote:
Originally Posted by Mooly View Post
You seem to be missing a resistor between pin 6 and pin 7.

The overall gain is set approximately by the ratio of the resistor between pin 6 and 7 and the resistor from pin 7 to ground (via its cap of course). No resistor gives unity gain (and it may not be stable like that).
If I place a 50K pot in place of the 7.5k is there a way to possibly use only this pot to turn the volume completely down? As you had mentioned "off" would be unity gain in this case, so I'm assuming no dice?

I'm leaning towards 50K because the distortion available is pretty nice.
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Old 13th February 2018, 03:29 PM   #28
Eps is offline Eps  United States
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Originally Posted by rayma View Post
If the eq is earlier in the circuit, there's more headroom. If the eq is later in the circuit, there's less noise.
Lower supply voltage gives less headroom.

If the eq is in the first stage, you could connect the recording tape head to that output,
instead of going through the second flat stage, if the first stage's gain is enough.
Really good to know! I have three questions about this. It seems as though introducing the second flat gain stage removed some HFs and made everything a bit muffled. Is that common and is there a common "makeup" of high frequencies remedy?"

Also, if I'm using a line level signal in conjunction with a DC bias (approx. 80 khz) would I normally need to go through something like a single pre-amp stage with NAB?

Lastly, when adding DC bias into the record head I would go post pre-amp and then add some caps and resistors to block the DC from going back into the pre correct?

Thank youuuuu
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Old 13th February 2018, 05:38 PM   #29
Mooly is offline Mooly  United Kingdom
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Squeezing maximum gain from one dual op-amp?
Quote:
Originally Posted by Eps View Post
If I place a 50K pot in place of the 7.5k is there a way to possibly use only this pot to turn the volume completely down? As you had mentioned "off" would be unity gain in this case, so I'm assuming no dice?

I'm leaning towards 50K because the distortion available is pretty nice.
You can not reduce the gain below unity (a gain of 1) for this configuration. A 50k pot would allow the total gain to be varied but you could well find that there is a minimum level beyond which things turn ugly and the circuit starts oscillating.
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Old 13th February 2018, 05:48 PM   #30
Eps is offline Eps  United States
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Originally Posted by Mooly View Post
You can not reduce the gain below unity (a gain of 1) for this configuration. A 50k pot would allow the total gain to be varied but you could well find that there is a minimum level beyond which things turn ugly and the circuit starts oscillating.
Yea that is what's going on so I'll have to figure out the minimum Ohms before oscillation on the pot and place a limiting resistor I suppose?
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