Hi everyone,
The title says it all really, I was toying with restoring a 70s stereo amplifier or building a gainclone. As the 70s amp has now gone higher than what I was willing to bid my mind has been made up, so gainclone it is 🙂
A few simple questions first, wanting to order the audiosector LM3875 dual mono basic kit and I have read the fantastic guide on how to assemble it and read alot of posts on here too.
With the basic kit am I right in thinking I just need to add a
300VA 230v to 2x22v (live in the UK)
and essentially I will have a working stereo power amplifier that will give me a power output roughly around 40w?
Obviously I will need chassis, heatsink, safety features etc.
But if that is the basic things I need, I would be able to work the rest out myself.
The only other question, I might have missed some threads, but what basic things would I need to build 2 monoblocks? I'm guessing 2xtransformers and 2xkits?
I think for now I'd like to keep it simple, just wondering for future upgrades.
Last question is about a preamp, would I be able to use line outputs from a stereo receiver for now until I looked in to a dedicated preamp?
Thanks in advance guys
My mouse is hovering on the buy it now buttons, but need confirmation first
The title says it all really, I was toying with restoring a 70s stereo amplifier or building a gainclone. As the 70s amp has now gone higher than what I was willing to bid my mind has been made up, so gainclone it is 🙂
A few simple questions first, wanting to order the audiosector LM3875 dual mono basic kit and I have read the fantastic guide on how to assemble it and read alot of posts on here too.
With the basic kit am I right in thinking I just need to add a
300VA 230v to 2x22v (live in the UK)
and essentially I will have a working stereo power amplifier that will give me a power output roughly around 40w?
Obviously I will need chassis, heatsink, safety features etc.
But if that is the basic things I need, I would be able to work the rest out myself.
The only other question, I might have missed some threads, but what basic things would I need to build 2 monoblocks? I'm guessing 2xtransformers and 2xkits?
I think for now I'd like to keep it simple, just wondering for future upgrades.
Last question is about a preamp, would I be able to use line outputs from a stereo receiver for now until I looked in to a dedicated preamp?
Thanks in advance guys
My mouse is hovering on the buy it now buttons, but need confirmation first
Hi thanks for the reply,
Speakers are currently Tannoy Mercury M3 specs;
PERFORMANCE
Recommended amplifier power : 10 - 90 Watts RMS
Continuous power handling : 60 Watts RMS
Sensitivity : 89dB (2.8Volts @ 1 metre)
Nominal impedance : 8 ohms
Frequency response : 35Hz - 20kHz +/- 3dB
But I am looking to change the speakers.
Having reread one of the threads and noticing my mistake, the basic kit and 2 300va 22vx2 windings will will give me a dual monoblock construction?
Speakers are currently Tannoy Mercury M3 specs;
PERFORMANCE
Recommended amplifier power : 10 - 90 Watts RMS
Continuous power handling : 60 Watts RMS
Sensitivity : 89dB (2.8Volts @ 1 metre)
Nominal impedance : 8 ohms
Frequency response : 35Hz - 20kHz +/- 3dB
But I am looking to change the speakers.
Having reread one of the threads and noticing my mistake, the basic kit and 2 300va 22vx2 windings will will give me a dual monoblock construction?
Hi,
300VA is way too much for an amplifier nominally rated 80W total.
150VA is still plently enough. Chips amps can drive 4 ohms, but
not very effectively unless you parallel them, even if 4 ohm rated.
A much smaller transformer can easily handle 40W music program
per channel, (programme = the peaks @40W are clipping, output
is not 40W rms, its typically much less on average for music).
Given your not making a laboratory sine wave generator and
continually running it at full power, or continually running
the music amplifier at full volume, i.e. just below the
point the programme clipping becomes objectionable,
in theory about 80VA would work, but personally I'd go
for 120VA for 8 ohm, and 200VA for paralled 4 ohm.
Your definition of dual monoblocks seems dual stereo.
Note all my VA numbers are for one transformer for
two channels, overkill for single chip amps is just
pointless. If your going to do it, with big heatsinks,
then simply also parallel up the chip amps.
Two monoblocks, with paralled LM3875 each,
would need 120VA transformers in each one,
and that is playing it very safe on the sizing.
rgds, sreten.
Also note in a stereo amplifier case, you can have one transformer with one
PS board for both channels, or dual PS boards for each channel, the latter
is more akin to monoblocks. Twin transformers is pointless in one case.
300VA is way too much for an amplifier nominally rated 80W total.
150VA is still plently enough. Chips amps can drive 4 ohms, but
not very effectively unless you parallel them, even if 4 ohm rated.
A much smaller transformer can easily handle 40W music program
per channel, (programme = the peaks @40W are clipping, output
is not 40W rms, its typically much less on average for music).
Given your not making a laboratory sine wave generator and
continually running it at full power, or continually running
the music amplifier at full volume, i.e. just below the
point the programme clipping becomes objectionable,
in theory about 80VA would work, but personally I'd go
for 120VA for 8 ohm, and 200VA for paralled 4 ohm.
Your definition of dual monoblocks seems dual stereo.
Note all my VA numbers are for one transformer for
two channels, overkill for single chip amps is just
pointless. If your going to do it, with big heatsinks,
then simply also parallel up the chip amps.
Two monoblocks, with paralled LM3875 each,
would need 120VA transformers in each one,
and that is playing it very safe on the sizing.
rgds, sreten.
Also note in a stereo amplifier case, you can have one transformer with one
PS board for both channels, or dual PS boards for each channel, the latter
is more akin to monoblocks. Twin transformers is pointless in one case.
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1x200 to 300 VA transformer,
and 2 kits.
"would I be able to use line outputs from a stereo receiver"
yess.
You can.
these things don''t really need a pre-amp.
a stereo audio taper will do just fine to adjust the volume.
and 2 kits.
"would I be able to use line outputs from a stereo receiver"
yess.
You can.
these things don''t really need a pre-amp.
a stereo audio taper will do just fine to adjust the volume.
Hi thanks for the reply,
I was reading this thread http://www.diyaudio.com/forums/audi...al-gainclone-kit-building-instructions-2.html and it mentions 22+22 300va winding.
The amp will always be driving 8R speakers. Power wise 40w is ample, I won't be running it close to the edge all the time. At the moment with my speakers and AV receiver I only really use around 10w.
So would the basic amp kit and a single 120VA 22Vx2 winding be enough? I know these sound like basic questions, but once I know the basics I can figure the rest out.
I think you might be right about the dual stereo.
My idea is;
1xWinding fed into 2xrectifier boards that then supply 2xamp boards, does this sound correct?
Thanks Mark
I was reading this thread http://www.diyaudio.com/forums/audi...al-gainclone-kit-building-instructions-2.html and it mentions 22+22 300va winding.
The amp will always be driving 8R speakers. Power wise 40w is ample, I won't be running it close to the edge all the time. At the moment with my speakers and AV receiver I only really use around 10w.
So would the basic amp kit and a single 120VA 22Vx2 winding be enough? I know these sound like basic questions, but once I know the basics I can figure the rest out.
I think you might be right about the dual stereo.
My idea is;
1xWinding fed into 2xrectifier boards that then supply 2xamp boards, does this sound correct?
Thanks Mark
This is what I am after, one transformer to dual PS to each channel
Your wording makes mine sound like I am speaking a different language 🙄
But this is basically what I am after, so I would need 1xsuitable winding and 1xbasic amp kit (consists of 2xPS PCBs and 2xAMP PCBs)?
Hi,
The best pot is dual linear with an added law faking resistors (google),
(It has much better channel matching than any audio taper pot.)
The "best" transformer is a big deal in production runs, but
in DIY it is really about price breaks, e.g. 150VA may cost
barely more than 120VA, 100VA could be much cheaper.
Chip amps (like op-amps) are not designed to need separate
PS boards, (just local decoupling), often in AV you'll find
lots of them all driven by one transformer and PS board.
There is no need to use two PS boards, and IMO the
Audio Sector approach sucks, as each board is dual,
so you only use both with two transformers, i.e. full
monoblocks, and monoblocks don't need dual boards.
YMMV but you don't need boards at all to build gainclones.
AudioSector are unduly complicating something simple.
rgds, sreten.
The best pot is dual linear with an added law faking resistors (google),
(It has much better channel matching than any audio taper pot.)
The "best" transformer is a big deal in production runs, but
in DIY it is really about price breaks, e.g. 150VA may cost
barely more than 120VA, 100VA could be much cheaper.
Chip amps (like op-amps) are not designed to need separate
PS boards, (just local decoupling), often in AV you'll find
lots of them all driven by one transformer and PS board.
There is no need to use two PS boards, and IMO the
Audio Sector approach sucks, as each board is dual,
so you only use both with two transformers, i.e. full
monoblocks, and monoblocks don't need dual boards.
YMMV but you don't need boards at all to build gainclones.
AudioSector are unduly complicating something simple.
rgds, sreten.
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Thanks sreten.
I am just trying to work out what I need at this point.
If I went along the lines of 1xTransformer to 1xRectifer feeding 2xAmps (it seems like a simpler way to do it) how would I determine what size transformer would be sufficient enough with some extra headroom.
As I mentioned before I am wanting to drive 2x8Ohm speakers with lets say 40w. The price of the 300VA 2x22V transformer is good at £30.
I think my knowledge is breaking down on how to chose the correct transformer.
Thanks for the information so far.
I am just trying to work out what I need at this point.
If I went along the lines of 1xTransformer to 1xRectifer feeding 2xAmps (it seems like a simpler way to do it) how would I determine what size transformer would be sufficient enough with some extra headroom.
As I mentioned before I am wanting to drive 2x8Ohm speakers with lets say 40w. The price of the 300VA 2x22V transformer is good at £30.
I think my knowledge is breaking down on how to chose the correct transformer.
Thanks for the information so far.
many kind of calculations can be done.
but brute force kind is what i prefer.
amplifier has losses, in the form of heat.
and must allso have more power input than it has to put "out".
You are looking for 2 amps, eatch should be pushing out 40 watt.
if everything in the world would be lossless, it would yield an ideal traffo of 80 VA.
the world is not ideal. least 2 times this is what i personally call bare minimum.
160 VA we are at.
I personally prefer to choose the next available size after this.
mostly that is 200 VA.
300 VA might be overkill, but won't hurt anything.
at all.
there is some scientific horror formula or what to calculate exactly what You need,
but besides costs, there is nothing on the world i know of would make it a better choice than a 200 VA rated toroid traffo.
but brute force kind is what i prefer.
amplifier has losses, in the form of heat.
and must allso have more power input than it has to put "out".
You are looking for 2 amps, eatch should be pushing out 40 watt.
if everything in the world would be lossless, it would yield an ideal traffo of 80 VA.
the world is not ideal. least 2 times this is what i personally call bare minimum.
160 VA we are at.
I personally prefer to choose the next available size after this.
mostly that is 200 VA.
300 VA might be overkill, but won't hurt anything.
at all.
there is some scientific horror formula or what to calculate exactly what You need,
but besides costs, there is nothing on the world i know of would make it a better choice than a 200 VA rated toroid traffo.
Thanks arty,
So in principle I just need something like this
Standard Range Toroidal Transformers: CM0225222: 225VA 230v to 2x22v
then feed it in to one rectifier PCB that will then drive 2 amplifier PCBs?
I would then end up with a stereo power amplifier? Although beit at around 40w?
If i bought the above audiosector LM3875 basic kit would I need to change any value components to use it with a
225VA 230v to 2x22v transformer?
After this I should be able to work it out, I just get confused on how to match a transformer to the use.
Thanks Mark
So in principle I just need something like this
Standard Range Toroidal Transformers: CM0225222: 225VA 230v to 2x22v
then feed it in to one rectifier PCB that will then drive 2 amplifier PCBs?
I would then end up with a stereo power amplifier? Although beit at around 40w?
If i bought the above audiosector LM3875 basic kit would I need to change any value components to use it with a
225VA 230v to 2x22v transformer?
After this I should be able to work it out, I just get confused on how to match a transformer to the use.
Thanks Mark
Hi,
Clueless overkill is stil clueless overkill, DIY it does no harm.
One AS PS board is a dual rectifier board for two amplifiers.
Its very OTT for a 40W per channel amplifier, and IMO
not that particularly well designed either.**
If you go full mono block (two cases) you can half populate
the two boards or buy the pretty pointless extra diodes
and caps kit so your running two already totally over
the top rectifier circuits in parallel for each channel.
In principle, not handwaving, (I'm not going to justify
the complicated issues*), you will be fine with a 100VA
transformer and wasting your time going over 150VA.
rgds, sreten.
* Matching a transformer to use for music is not simple.
Any pretence it is, is simply oversimplistically wrong.
** The board is totally unnecessary for a bridge rectifier
on its back clamped to the case, and connected to the
transformer, with 10uF wired across the output.
Clueless overkill is stil clueless overkill, DIY it does no harm.
One AS PS board is a dual rectifier board for two amplifiers.
Its very OTT for a 40W per channel amplifier, and IMO
not that particularly well designed either.**
If you go full mono block (two cases) you can half populate
the two boards or buy the pretty pointless extra diodes
and caps kit so your running two already totally over
the top rectifier circuits in parallel for each channel.
In principle, not handwaving, (I'm not going to justify
the complicated issues*), you will be fine with a 100VA
transformer and wasting your time going over 150VA.
rgds, sreten.
* Matching a transformer to use for music is not simple.
Any pretence it is, is simply oversimplistically wrong.
** The board is totally unnecessary for a bridge rectifier
on its back clamped to the case, and connected to the
transformer, with 10uF wired across the output.
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Excuse me if it seems like I am coming across as ignorant, as I really am not.
Like I have said before, my knowledge stops at transformers and how to match them.
On my reference sample (that I like I can push my speakers to 4.84v which should be around 24 watts after this it is pointless).
My sample;
https://www.youtube.com/watch?v=lpB1yR9Sz9Y
What transformer would be ample enough to deliver this, as I keep hearing different values.
If I could do the gainclone to achieve these levels I would be more than happy 🙂
Like I have said before, my knowledge stops at transformers and how to match them.
On my reference sample (that I like I can push my speakers to 4.84v which should be around 24 watts after this it is pointless).
My sample;
https://www.youtube.com/watch?v=lpB1yR9Sz9Y
What transformer would be ample enough to deliver this, as I keep hearing different values.
If I could do the gainclone to achieve these levels I would be more than happy 🙂
Last edited:
I should also note, that at these levels it is neighbor disturbing and that is the maximum I would ever push it.
The reason I chose that clip, is classical is too variable rock is too constant but this clip my ears say no more but leave me with a big grin factor.
If there is a formula for working out transformer parameters based on output I would be happy to study it myself and do the math myself, if someone could present me with the formula.
The reason I chose that clip, is classical is too variable rock is too constant but this clip my ears say no more but leave me with a big grin factor.
If there is a formula for working out transformer parameters based on output I would be happy to study it myself and do the math myself, if someone could present me with the formula.
Hi,
Approximately decent music spends 80% of the time below 20%
of the peak value, known as the crest factor. So at music clipping
average output is not rms clipping values, it is much lower.
A simple and not wrong view is that an amplifier transformer, needs
about its rated output in VA, if well matched to the loudspeaker.
I'm not going to argue the toss, just that this is ~ the tipping point.
rgds, sreten.
Approximately decent music spends 80% of the time below 20%
of the peak value, known as the crest factor. So at music clipping
average output is not rms clipping values, it is much lower.
A simple and not wrong view is that an amplifier transformer, needs
about its rated output in VA, if well matched to the loudspeaker.
I'm not going to argue the toss, just that this is ~ the tipping point.
rgds, sreten.
Be careful with your voltages.
A 230:22+22Vac transformer running on the UK's nominal 240Vac will give a higher voltage.
The transformer regulation also gives an higher voltage than the "fully loaded" specification.
Your no load output voltage is:
Vout = Vmains / Vpri * Vsec * {1+regulation}
The maximum output voltage of a 6% regulation 230:22+2Vac transformer on the maximum UK voltage of 253Vac will be 252/230*22*1.06 = 25.65Vac
The peak voltage will be 36.28Vpk
Subtract a bit for a no load diode drop and you have a potential for ~35.8Vdc across your smoothing capacitor.
DO NOT use 35V caps. Use either 40V or 50V capacitors.
A 230:22+22Vac transformer running on the UK's nominal 240Vac will give a higher voltage.
The transformer regulation also gives an higher voltage than the "fully loaded" specification.
Your no load output voltage is:
Vout = Vmains / Vpri * Vsec * {1+regulation}
The maximum output voltage of a 6% regulation 230:22+2Vac transformer on the maximum UK voltage of 253Vac will be 252/230*22*1.06 = 25.65Vac
The peak voltage will be 36.28Vpk
Subtract a bit for a no load diode drop and you have a potential for ~35.8Vdc across your smoothing capacitor.
DO NOT use 35V caps. Use either 40V or 50V capacitors.
Hi Andrew thanks,
Really didn't think about off load voltage and the output voltage rising at all, told you I am a novice.
Since this is my first amp, I'd like to start by not having to change component values yet and learn by using with what comes in the kit.
Ok so if I have done my sums right, I am wondering if a 230:18+18Vac (160Va 10%reg) would be ok?
252/230*18*1.10=21.69Vac, and I have worked the peak to be 32.54Vpk.
Would this be suitable for 35V caps or am I close to the limit still?
Really didn't think about off load voltage and the output voltage rising at all, told you I am a novice.
Since this is my first amp, I'd like to start by not having to change component values yet and learn by using with what comes in the kit.
Ok so if I have done my sums right, I am wondering if a 230:18+18Vac (160Va 10%reg) would be ok?
252/230*18*1.10=21.69Vac, and I have worked the peak to be 32.54Vpk.
Would this be suitable for 35V caps or am I close to the limit still?
Mark
Not for being offtopic... But you know, that one lm4780 can be used as a stereoamp., just very fine 🙂
Been using one for years, for my "normal" listning... Really a very good sounding amp!
Happy building...
Regards;
Jesper.
Not for being offtopic... But you know, that one lm4780 can be used as a stereoamp., just very fine 🙂
Been using one for years, for my "normal" listning... Really a very good sounding amp!
Happy building...
Regards;
Jesper.
I have just looked at the BOM and have noted it comes supplied with 50V smoothing caps.
LM3875 Dual Mono Classic Kit 1PCB set, consisting of 2 amplifier and 2 rectifier boards. 2LM3875TF 422k 0.5w compact carbon resistors 2220R 0.5w compact carbon resistors 2680R 0.5w compact metal film resistors 41500uF 50v Panasonic FC Capacitors 16MUR860 OnSemi Diodes 210uF Panasonic FC Capacitors
So if I am right in assuming a 230:18+18Vac Transformer would be fine to use and I still have headroom to use a higher secondary voltage?
I think due to my lack of knowledge I'd be happy at achieving 25Wrms Per channel from this.
Using Vpk into 8Ohms would give me a potential maximum of 58.80 Watts per channel?
This doesn't seem correct but I suppose it ties in with a 160VA transformer or am I confusing myself?
LM3875 Dual Mono Classic Kit 1PCB set, consisting of 2 amplifier and 2 rectifier boards. 2LM3875TF 422k 0.5w compact carbon resistors 2220R 0.5w compact carbon resistors 2680R 0.5w compact metal film resistors 41500uF 50v Panasonic FC Capacitors 16MUR860 OnSemi Diodes 210uF Panasonic FC Capacitors
So if I am right in assuming a 230:18+18Vac Transformer would be fine to use and I still have headroom to use a higher secondary voltage?
I think due to my lack of knowledge I'd be happy at achieving 25Wrms Per channel from this.
Using Vpk into 8Ohms would give me a potential maximum of 58.80 Watts per channel?
This doesn't seem correct but I suppose it ties in with a 160VA transformer or am I confusing myself?
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