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Chip Amps Amplifiers based on integrated circuits

Squeezing maximum gain from one dual op-amp?
Squeezing maximum gain from one dual op-amp?
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Old 13th February 2018, 06:00 PM   #31
Mooly is offline Mooly  United Kingdom
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Squeezing maximum gain from one dual op-amp?
That is about all you can do really.

If you did have a specific gain in mind then there is a trick we can use to stabilise the opamp but it would be trial and error and would really need an oscilloscope to observe the effects.

If you connect a resistor between the two opamp inputs then that allows you to use a lower gain while 'fooling' the opamp into thinking its working at higher gain.

The additional resistor doesn't alter your set gain.

Noise Gain vs. Signal Gain | Analog Devices
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Old 13th February 2018, 06:19 PM   #32
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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.
Actually I was mistaken, at above 47k on the 50k pot I get into oscillation/RF bleed maybe?? At least it has this RF type sound to it. I added a 47K in parallel and and that took care of it.

However, when I attach the remaining leg of the pot to ground oscillation will still begin around 40k on the pot. What is happening here? Pot should be ungrounded?
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Old 13th February 2018, 07:01 PM   #33
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Squeezing maximum gain from one dual op-amp?
It is hard to say for sure what is happening. When dealing with a high gain circuits you will find the layout and wiring becomes quite important to maintaining stability.

If the pot is a metal bodied one then ground the outer metalwork may well help stray pickup.
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Old 13th February 2018, 07:19 PM   #34
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Originally Posted by Mooly View Post
It is hard to say for sure what is happening. When dealing with a high gain circuits you will find the layout and wiring becomes quite important to maintaining stability.

If the pot is a metal bodied one then ground the outer metalwork may well help stray pickup.
Ok cool, I'll play with that. Do you think it's a problem to leave the pot ungrounded or should that extra leg be tied with the center leg?

I was messing with placing a resistor between the two inputs as you mentioned while the cassette was playing back and it actually ended up recording some saw wave feedback to the cassette tape when I played it back again haha! aka the cassette was playing back and recording through the same head
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Old 13th February 2018, 07:29 PM   #35
Mooly is offline Mooly  United Kingdom
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Squeezing maximum gain from one dual op-amp?
If it is a traditional metal pot then try grounding the body. The third leg can be either left floating or tied to the middle one, it doesn't really matter electrically, and it shouldn't affect any stray pickup either.

If in doubt try both methods, its only a seconds job to tag them together.

Have you considered using a more conventional opamp ?... these dedicated consumer types aren't always very amenable to incorporating into other circuits and can be temperamental and finicky to get satisfactory results from them.

Although they are basically an 'opamp' they are often designed specifically for one specific consumer application such as a tape head preamp.
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Old 27th February 2018, 06:22 PM   #36
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Originally Posted by Mooly View Post
Have you considered using a more conventional opamp ?... these dedicated consumer types aren't always very amenable to incorporating into other circuits and can be temperamental and finicky to get satisfactory results from them.

Although they are basically an 'opamp' they are often designed specifically for one specific consumer application such as a tape head preamp.
Yea I just like this gritty sound

Guessing I could get similar gritty sound from something else but pretty happy already!
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Old 27th February 2018, 06:31 PM   #37
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If it is a traditional metal pot then try grounding the body. The third leg can be either left floating or tied to the middle one, it doesn't really matter electrically, and it shouldn't affect any stray pickup either.
It's actually mostly plastic so I suppose should be good thx.

I have been attempting to create this bias erase circuit (attached) and it burns up my pnp and npn every time. Im using BC337 and BC327 as stand ins. I would guess that wouldn't cause the issue? Also just get more crazy osc as they die.... Does it matter that I'm using a bipolar 1uf in place of the ceramic 1uf to ground in this case?

I'm attempting to use a regular mono playback head to erase with. Not sure if that is where my problem lies creating some feedback and burning out both transistors?
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Old 27th February 2018, 07:23 PM   #38
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Squeezing maximum gain from one dual op-amp?
The characteristics of the erase head are a fundamental part of the original design with everything being designed around it. So differences in self inductance, DC resistance and so on will all drastically alter things.

It is possible the 1uF could play a part. A ceramic cap is ideally suited due to its low losses at high frequencies. A film cap should OK as well... but all that assumes the design suits the head you are using... which tbh it probably doesn't.
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