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Old 25th February 2009, 06:00 PM   #1
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Default Simple pre-amp for gainclone help req'd.

Hi I am a noob of sorts, and I wanted to share my project to see if I can avoid any stupid pitfalls...

I have ordered a LM3875 chip-amp kit from audiosector. I have 6 Ohm Tannoy dual concentric 91dB speakers so I will get a 20v 300VAC transformer for dual mono build.

That much I think is fairly straightforward.

What I want to do is build a 400mm w. x 250mm d. x 100mm h. wooden case for a stereo that includes the power amp, a DACT type stepped attenuator (from eBay Hong Kong), my DAC PCBs (Beresford) and transformer, my airport express (uncased, with an external aerial) and a 7 input selector switch that switches between wi-fi, coax 1, coax 2, optical, analogue 1, analogue 2 & Phono (which I'll make or fit a pre-amp for later).

My issue is that I want to make the simplest form of pre-amp for the chip-amp...basically an attenuator for volume and a selector switch that switches between the digital inputs of the DAC and 3x analogue inputs. Can anybody suggest the value of that attenuator, and if the pre-amp needs to do anything else, other than a simple switch. Are the voltage levels correct to go from source straight into the power amp via the attenuator, for example?

Many thanks in advance
Lucas
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Old 25th February 2009, 06:26 PM   #2
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That is correct. Many of us use just a log (aka audio taper) potentiometer, such as a Noble or Alps, on the input to a power amp and nothing more.
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Old 25th February 2009, 06:26 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
use a buffer, gain=1times (+0dB), immediately after the attenuator.

Your Tannoys, 91dB, will require less than 1Vac signal to sound quite loud.
If the chipamp has 28times gain (+28dB) you will need a lot of attenuation between 2Vac digital sources and the chipamp.
You will need a bit less attenuation for 300mVac sources eg FM tuner.

There is no need for any gain in the pre-amp.

if you decide to use a passive pre-amp then choose a 10k attenuator, but ensure all your sources can drive a 10k load + cable capacitance adequately.

If you use a buffer you can choose any attenuator from 10k to 100k.

Check your Tannoy impedance. 6ohms sounds a bit odd.
The big DCs are 8ohm or 16ohm.
The small DCs are usually 8ohm but some use dual bass drivers bringing the impedance down to 4ohm. These dual bass driver speakers should be described as 4 to 8 ohm.
It's just possible that Tannoy wound them as 12ohm drivers to give the 6ohm you quoted.
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Old 26th February 2009, 12:40 PM   #4
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Thanks for the replies.

Andrew, the impedance is described as 6 Ohm nominal, which I think means that it varies a lot at various frequencies, and averages out at about 6, hence the word "nominal". In reality it's like you say - between 4 and 8 Ohms. There are indeed 2x 8" bass drivers, one of which has a 1" tweeter mounted inside it.

So, this buffer...I hear a lot about tube-based buffer pre-amps. If I have different levels coming in, do I need a different buffer on each channel? My DAC, for example, puts out 1.35 volts. My CD player, about 2 volts. I'd like 3 analogue inputs ideally - one for the DAC, one for an analogue source, and another for phono. If they all require different levels of buffering, how does that work?

Can anybody point me in the direction of a design I could follow - I am not an electrical engineer, obviously, but can recognise and follow good advice when I hear it, and that's the basis of my hobby at this point.

Many thanks
Lucas
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Old 26th February 2009, 12:40 PM   #5
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"if you decide to use a passive pre-amp then choose a 10k attenuator, but ensure all your sources can drive a 10k load + cable capacitance adequately."

Now, I wouldn't have a clue how to do this...I don't really understand what all that means - make sure my signals are strong enough for this pot?
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Old 26th February 2009, 02:14 PM   #6
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The best option is Trial-&-Error. Buy one pot each with 10 k, 22(20) k, 47(50) k and check, if you hear a difference. If you do, choose the best sounding one. If you don't, stick with 10 k.

The alternative is to get a schematic of your source or open it up and analyze the output stage.
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Old 26th February 2009, 05:12 PM   #7
AndrewT is offline AndrewT  Scotland
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You don't usually need a buffer on each or indeed any input.
You do need a buffer on each output if the equipment does not already have a suitable drive for the cables and next stage impedance.

A passive pot has an output impedance that varies from zero to [Rs + POT value] / 4
This output impedance should be <1000ohms and preferably around 50 to 200ohms to drive long cables and low impedance inputs.
A 10k pot + 200ohm Rs will have an output impedance between 0r0 and 2550r.
A 50k pot changes the output impedance range to 0r0 to 12550r

Neither are great at driving capacitance.
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Old 10th September 2009, 12:59 AM   #8
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My problem is that my Beresford pre-amp has a direct output (2Vac) and a headphone output, with a volume control. The direct output put waaay to much into the chip amp and causes hideous distortion in serious danger of destroying my speakers at all but the tiniest of volumes at source. I use the headphone output from the DAC, and set it at halfway (5 of 10) which gives me the right level of input to the amp.

Is there any easy way I can get my amp to accept the direct out from the DAC?

Many thanks
Lucas
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Old 10th September 2009, 04:31 AM   #9
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Don't ever act on my advice, I'm not what you'd call an engineer.

But I believe this is as simple as placing a series resistor on your line-level in.

Last edited by raypalmer; 10th September 2009 at 04:32 AM. Reason: jargon
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Old 10th September 2009, 08:43 AM   #10
AndrewT is offline AndrewT  Scotland
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fit an attenuator.
It can be a cheap carbon or plastic tracked potentiometer, or a stepped resistor string or a digital volume control or a lightspeed.

All of these will reduce your input to the power amp to give acceptable SPL (volume) in the listening room.

If you find that the attenuator is at the very lowest end most of the time and that this makes small adjustments difficult, then consider adding a fixed attenuator in line with the adjustable version.
The fixed attenuator will consist of two resistors for each channel.This can be fitted inside an RCA plug and thus is removeable/insertable without needing any modification to any of your equipment.
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