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Old 25th Jan 2016, 11:47 am   #1
sp10mk11
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Default Increasing The Gain on a Pre-amp

Hello attached is a circuit of a pre-amp I have , I would like to increase its output does this look feasible, I was looking at the base resistor 2.2K on Q603L and possibly lowering its value to increase the signal going to the power amp.

All thoughts welcome

Gary
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Old 25th Jan 2016, 11:55 am   #2
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Default Re: Increasing The Gain on a Pre-amp

That won't make a difference. Q603L is an emitter follower. Its gain is unity. There is a little loss in that resistor given the bias resistors etc, but not much. And the resistor performs an important function - it is there to stop RF oscillation.

The gain comes from the three transistor network before Q603L and the feedback networks etc appear to be off the bottom of the picture.

David
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Old 25th Jan 2016, 12:00 pm   #3
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Default Re: Increasing The Gain on a Pre-amp

If you really do need more gain it makes more sense to add an op-amp after the output. Very few components are needed, the cost is vey low and you can set the gain to whatever you like.
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Old 25th Jan 2016, 12:45 pm   #4
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Default Re: Increasing The Gain on a Pre-amp

I only want to increase the output by about 6db.
Thanks for tip Paul some where I have a velleman kit for a line amp............it could take till doomsday to find it but that would be ideal as it has variable gain and I could put it inbetween the power/pre amp connectors to experiment.

Thanks chaps, a hunting I will go!

Gary
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Old 25th Jan 2016, 1:24 pm   #5
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Default Re: Increasing The Gain on a Pre-amp

Try a small reduction of the value of R622 (or parallel a preset resistor across it).
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Old 25th Jan 2016, 1:27 pm   #6
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Default Re: Increasing The Gain on a Pre-amp

If you look at the datasheet for something like a TL072 it will contain circuits for variable gain amps.
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Old 25th Jan 2016, 4:32 pm   #7
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Default Re: Increasing The Gain on a Pre-amp

Quote:
Originally Posted by sp10mk11 View Post
I only want to increase the output by about 6db.
Thanks for tip Paul some where I have a velleman kit for a line amp............it could take till doomsday to find it but that would be ideal as it has variable gain and I could put it inbetween the power/pre amp connectors to experiment.
If its the kit I am thinking of, it doesn't have variable gain. What it has is a pair of preset pots on the output. Not the best and most elegant way.
The good news is that you can tweak the gain very easily by the simple measure of different feedback resistors. If I remember correctly there is information supplied in the destructions to do this. You can also adjust input resistance to suit your purposes in the same manner.
I'd also not use the el cheapo electrolytic coupling capacitors and replace them with plastic film jobs of a suitable value. 1 uF to 2.2uF should be plenty and at that low working Voltage they aren't too big to squeeze on the board.
Nevertheless its a nice neat PCB and the use of an 8 pin DIL socket also means you can use a lower noise "better" op amp than the supplied TL072. Not that the TL072 is bad, it's not.
Unusual to need extra gain on a modern amp though.
What's the purpose of the gain stage? What are you wanting to amplify?

A.
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Old 25th Jan 2016, 4:37 pm   #8
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Default Re: Increasing The Gain on a Pre-amp

An afterthought, I make use of the inexpensive babani Books paperback on preamplifiers and filter circuits by R A Penfold. Maplins amongst others sell it. I have had my fivers worth and more from it.
Explains the uses of op amps and how to configure the common circuits. And also has Some very useful circuits ranging from piezo cartridge preamps, a couple of magnetic RIAA stages and passive and active tone controls and filters.
really useful if, like me, you need to remind yourself of the basics.

Andy.
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Old 25th Jan 2016, 4:57 pm   #9
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Default Re: Increasing The Gain on a Pre-amp

The National Semiconductors Linear Handbook has all you need (and more) and can be downloaded free.
 
Old 25th Jan 2016, 6:24 pm   #10
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Default Re: Increasing The Gain on a Pre-amp

Thanks to every one who is contributing on this.
The amplifier is a 1970s integrated amplifier that I have been playing with, and at times I feel the pre-amp is not putting enough signal into the power amp section, hence my request for help on raising the output of the pre-amp slightly, just say it gives 800mv I would like to increase it to 1v for example
Gary
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Old 25th Jan 2016, 6:49 pm   #11
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Default Re: Increasing The Gain on a Pre-amp

Ok Gary.

Daft question maybe,
Have you measured the amps sensitivity against output?
I'm assuming you have a scope or a means of measuring the output power versus input Volts.
messing about with the interstage gain will upset the amp. Stuff like feedback will be out of kilter I imagine and you may actually overload the stages.
Like wise, depending on where the Volume pot is in the circuit, amplifying the source prior to the amps line inputs could possibly overload the input stage.
Just something to think about Gary. If the input stages aren't giving enough drive to the power amp section then I'd start thinking the amp had a fault.
Do you know what the specs are on it? i.e. input sensitivity in mV to achieve rated power output.
If its because you have a RIAA preamp thats got a bit of a weedy output or some tuners were a bit low output then I can understand wanting to give it a bit of a lift.
One things for sure, if a CD player with a 2V output cant drive the amp to full output then I'd think there was something wrong.
Sorry if that's not what you want to hear.
A.
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Old 25th Jan 2016, 9:22 pm   #12
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Default Re: Increasing The Gain on a Pre-amp

Hello Bikernut
I know what you are saying and I understand, I don't have a CD to try out for full output.
But yes I feel that say a Tuner or phono is a bit low, I have checked all the voltages etc and all checks out, its just that I think it needs a few more millivolts output to the power amp, I tried using the line section on a mixer between the pre-out and power in I and of I can over drive the power amp input with that, but it does show me that a little more gain from the pre-amp would sort it, to use an op-amp might be a bit too much.
If you look at the circuit diagram you can see volume control/balance controls are at the output of the pre-amp.
So I was thinking maybe I could tweek the pre-amp to achieve this, Restoration73 has suggested reducing the value of R622 (8.2K) this I will try.
I have to have a fiddle
Gary

Ps yes I can measure the output, I could supply a line level 1k signal and measure what the pre-amp is putting out
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Old 25th Jan 2016, 10:09 pm   #13
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Default Re: Increasing The Gain on a Pre-amp

Ps yes I can measure the output, I could supply a line level 1k signal and measure what the pre-amp is putting out[/QUOTE]

Ok well I think I can see the issue now and its one I had with my Arcam Delta280 tuner which puts out about 250mV and my amps generally run with 500mV or 1V sensitivity which is fine with CD.
So I also considered a simple op amp step up. I figured it was a simple fix and if you use suitable quality coupling caps (plastic film instead of electrolytic) I didn't think there would be that much noticeable difference, if any.
I reckon you have nothing to lose by trying the opamp approach but with the gain adjusted via the feedback network and say 4x gain. A decent hi fi amp should have enough headroom on its inputs to cope.
The same sort of circuit should also be fine to step up a moving magnet cartridge if you need a bit more. Just adjust the input resistors to give 47k, you'll find its easier to get 50k and that's close enough. That's also fine as an input resistance for a line source if you don't want to build 2 step up amps.
I must look it up but I seem to remember some magnetic cartridges were closer to 2mV output than the standard 5mV. Shures spring to mind here.

A.
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Old 26th Jan 2016, 12:32 am   #14
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Default Re: Increasing The Gain on a Pre-amp

A half decent dual audio opamp can be had for a few quid or less and can be set for any gain from unity upwards. It could go either at the front or between the preamp and main amp. It'll work far better than any "simpler" circuit.
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Old 26th Jan 2016, 9:27 am   #15
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Default Re: Increasing The Gain on a Pre-amp

If you are lacking output due to a low source level, the correct place to put the extra gain is at the source, not between preamp and power amp. Each subsection of a well designed unit or system will be designed to output as much signal swing as the following stage can accept. Adding extra gain midway will increase the level just the same but also reduce the dynamic range as the preamp noise will be amplified, yet the power amp will be overloaded before the headroom in the preamp is used.

When experimenting with the mixer, if it was professional kit it would likely be capable of +4dBu instead of -10dBV which is the normal interconnect level on consumer equipment, therefore able to drive the power amp well into overload.
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Old 26th Jan 2016, 9:58 am   #16
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Default Re: Increasing The Gain on a Pre-amp

Right well I have a bit of time and shall try and find the Velleman kit to have a play with, as tweaking the pre-amp doesn't seem a good idea.

I will get back
Gary
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Old 26th Jan 2016, 2:56 pm   #17
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Default Re: Increasing The Gain on a Pre-amp

Quote:
Originally Posted by sp10mk11 View Post
just say it gives 800mv I would like to increase it to 1v for example
Gary
You would not be able to hear a change as small as this, even a gain of x2 is difficult to detect outside direct A/B conditions. Additional gains of x10 are readily noticable, anything less and its of dubious benefit.
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Old 26th Jan 2016, 3:21 pm   #18
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Default Re: Increasing The Gain on a Pre-amp

The question is, is it loud enough as is?
 
Old 26th Jan 2016, 5:03 pm   #19
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Default Re: Increasing The Gain on a Pre-amp

MerlinMaxwell
In answer to your question, it is probably is loud enough it probably me having a bad day and getting an idea in my head that until I try a few things out it wont go away
Studio263
Sorry I was just giving those figures as examples
Gary
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Old 26th Jan 2016, 6:25 pm   #20
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Default Re: Increasing The Gain on a Pre-amp

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Originally Posted by merlinmaxwell View Post
The question is, is it loud enough as is?
Aha!
Thats the 64 dollar question innit?


Mind you if it means you don't have to fiddle about with the volume control when switching between sources, theres a reason.
I'd say go for it Gary, build the velleman kit on the PCB as supplied. Adjust the output pots until you get the gain you need. Measure that amount of gain (Vout/Vin) and then put feedback resistors in to get the desired gain (use veropins in this position so its easy to substitute resistors). The advantage of this is you will get the best signal to noise performance from the opamp amongst other desirable stuff. And i'd still substitute the electrolytic coupling caps for polywotsits no matter what.
Could be fun just playing with it.
A.
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