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
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
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.
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.
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
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
"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?
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?
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.
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.
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
Is there any easy way I can get my amp to accept the direct out from the DAC?
Many thanks
Lucas
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.
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.
Thanks for the replies. I actually have a stepped attenuator on the amp. The problem is that of the 24 stages on the attenuator, I have to have is set to "1" or "2" to get a volume that doesn't threaten to blow my speakers. I followed the prevailing advice when fitting it. It is rated at 10K. Is that the problem?
Many thanks
Lucas
Many thanks
Lucas
Hi everybody
I'm sorry for being such a noob, but I'm still really confused about what to do. My stepped attenuator is a set of resister choices, but I'm not sure whether it is wired to the output of the amp or the input. I am pretty sure it's the output.
My Beresford DAC has two output choices. First is direct out, like a CD player, and second is headphone circuit out, with RCAs, using the headphone volume control (which is how I use it now). I want to use the direct out, so that I can bypass the headphone circuit that puts extra noise into the system, but with the direct out, the sound is very distorted, even at a fairly low volume level. I am guessing that the voltage is too high. Is that right? If my stepped attenuator were on the input of my amp, it would provide the required resistance to lower the voltage, right? Or maybe the values are wrong. What values should I try at the input to lower the voltage to a good level? What is the desired level for an LM 3875 chip amp outputting to 6 Ohm speakers?
Many thanks and sorry or being such a dunce.
Lucas
I'm sorry for being such a noob, but I'm still really confused about what to do. My stepped attenuator is a set of resister choices, but I'm not sure whether it is wired to the output of the amp or the input. I am pretty sure it's the output.
My Beresford DAC has two output choices. First is direct out, like a CD player, and second is headphone circuit out, with RCAs, using the headphone volume control (which is how I use it now). I want to use the direct out, so that I can bypass the headphone circuit that puts extra noise into the system, but with the direct out, the sound is very distorted, even at a fairly low volume level. I am guessing that the voltage is too high. Is that right? If my stepped attenuator were on the input of my amp, it would provide the required resistance to lower the voltage, right? Or maybe the values are wrong. What values should I try at the input to lower the voltage to a good level? What is the desired level for an LM 3875 chip amp outputting to 6 Ohm speakers?
Many thanks and sorry or being such a dunce.
Lucas
Does your DAC not have an adjustable RCA output?
The attenuator goes between DAC and amplifier.
A 10k attenuator is okay, but it should replace the 22k resistor from amplifier input to ground. If you simply put it in parallel, you lower the overall impedance the DAC has to drive. That increases the DAC's output HP filter frequency and can lead to increased distortion as well.
The attenuator goes between DAC and amplifier.
A 10k attenuator is okay, but it should replace the 22k resistor from amplifier input to ground. If you simply put it in parallel, you lower the overall impedance the DAC has to drive. That increases the DAC's output HP filter frequency and can lead to increased distortion as well.
Yes my DAC does have an adjustable phono out, utilising the headphone circuit, but it is a touch noisy, so I'd rather not go through it. At the moment I have the volume at halfway, which creates an OK signal level for the amp, but I don't get why the direct out is so ridiculously over the top for a chip amp. Stanley Beresford told me by email that the variable output puts out up to 1.35v and the direct out is more. I found that the mid setting of the pot gives me a reasonable level, but I'd like to be more accurate about it. I can't measure the ins and outs because my auto ranging DC multimeter is darting around like crazy and seems useless to me.
Regarding the attenuator I put in, I followed forum instructions to the letter, replacing the 22k resistors and all. It all works a treat apart from the discrepancy between signal output of the DAC and input tolerance of the amp. What to do? How would I go about measuring the output of my DAC's direct out and the amp's ideal input signal?
Many thanks
Lucas
No, no and no. Sorry. I guess that's what I need to do. Unfortunately an electrician borrowed my multi-meter instructions and lost them, so I have this Uni-T auto ranging thing that seems like a piece of crazy crap to me, darting around like an idiot, and I have no way of working out what's wrong with it.
Incidentally, it's not just my DAC's direct out. I've now tested it with a tuner and a cd player and it is distorting like a mother even at lowest volume levels. Why is my amp so sensitive?
Many thanks
Lucas
buy a cheap DMM that has both 200mVac and 200mVdc scales, as well as the higher voltage scales.
Fit a temporary fixed attenuator in each signal line:-
100k in series with the signal line and after that a 10k from signal line to signal ground. Or any pair of resistors that have a ~ten to one ratio somewhere in the range 22k+2k to 100k+10k. Both channels can be different.
This will reduce your output signal by approx 10times (~-20dB). It will also filter your treble slightly depending on what is fitted inside your chip power amp.
This will allow the chipamp to play at a sensible volume without overloading and help tell us where your fault is likely to lie.
Fit a temporary fixed attenuator in each signal line:-
100k in series with the signal line and after that a 10k from signal line to signal ground. Or any pair of resistors that have a ~ten to one ratio somewhere in the range 22k+2k to 100k+10k. Both channels can be different.
This will reduce your output signal by approx 10times (~-20dB). It will also filter your treble slightly depending on what is fitted inside your chip power amp.
This will allow the chipamp to play at a sensible volume without overloading and help tell us where your fault is likely to lie.
Because you have assembled it incorrectly or because you have no pre-amp to control the input signal.
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