How to reduce a preamp output voltage? - diyAudio
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Old 10th March 2015, 11:18 AM   #1
Stozzer is offline Stozzer  United Kingdom
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Default How to reduce a preamp output voltage?

Hi Folks,

I have a preamp that outputs at a maximum of 3 volts.

But my amp recommends not using a preamp due to the output signal voltage being too high.

I would like to reduce the maximum output voltage of the preamp to match the maximum of the amp (which below is marked as 0.520v)

How would i go about this, is it as simple as placing a metal film resistor in the signal path which is routed to ground?

Or perhaps creating a potential divider for each channel out of resistors and outputting from the mid point?

Below is the specs of the amp.

Click the image to open in full size.

I should mention that the reason why i want to use a preamp, is because i would like balance, treble, bass and volume control which is provided very conveniently in a Hi-hi preamp.

Last edited by Stozzer; 10th March 2015 at 11:59 AM.
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Old 10th March 2015, 12:16 PM   #2
Stozzer is offline Stozzer  United Kingdom
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I am thinking something like this?

Click the image to open in full size.

Last edited by Stozzer; 10th March 2015 at 12:21 PM.
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Old 10th March 2015, 12:23 PM   #3
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Yes, you can basically use a voltage divider if you do not feel comfortable modifying the preamp for reduced gain. You should, however, have somewhat of an idea of both the preamp's load driving capabilities and the power amp's expected maximum source impedance (something like a 4k7/1k divider should not pose much of a problem for many preamps but would be somewhat shy of the maximum 600 ohm source impedance demanded by some Class D amps).
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Old 10th March 2015, 12:29 PM   #4
wwenze is offline wwenze  Singapore
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1) Potential divider at the INPUT
2) Reduce gain

Or both.

Add zener clamp if you're still paranoid.
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Old 10th March 2015, 12:37 PM   #5
Stozzer is offline Stozzer  United Kingdom
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The Preamp kit i am using uses Phillips 1524A chip if it helps with the answer?

https://www.quasarelectronics.co.uk/...s/tda1524a.pdf
https://www.quasarelectronics.co.uk/...t-kit/1070.pdf
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Old 10th March 2015, 01:05 PM   #6
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TDA1524, ugh. What a noisy bugger. I don't think you can reduce gain with this one, so voltage divider it is. Minimum load is like 2k, so 4k7/1k should work fine from that side. Hiss levels may still be annoying (100 V / 5.7 + 32 dB ~= 700 V) and as such may eventually prompt replacement of preamp. A standard HiFi-grade preamp exhibits far lower noise, with 12.5 V (about 1/8 of yours) being considered run of the mill.

What's the power amp?
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Old 10th March 2015, 01:11 PM   #7
AndrewT is offline AndrewT  Scotland
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Everybody else is happy using a volume control.

Is that not good enough for your operators?

If not, then look at reducing excess gain rather than wasting what has already been amplified to too high a level.
i.e. throw away the preamp !
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regards Andrew T.
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Old 10th March 2015, 01:23 PM   #8
Stozzer is offline Stozzer  United Kingdom
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The power amp is this:

http://sure-electronics.net/download..._Ver1.0_EN.pdf
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Old 10th March 2015, 01:28 PM   #9
Stozzer is offline Stozzer  United Kingdom
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sgrossklass could you suggest a suitable preamp kit which would be less noisy, or at least the model of IC I should be looking for in a decent preamp?
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Old 10th March 2015, 01:39 PM   #10
Stozzer is offline Stozzer  United Kingdom
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That's a great point AndrewT, i can put an extra resistor in series with the volume pot to force the maximum possible voltage to be lower.

Last edited by Stozzer; 10th March 2015 at 01:47 PM.
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