Andrew,
just an idea: lamps instead of the heavier bleeders. With lamps, at their working voltage, one can have at least 5 times the low voltage resistance. A stack of three matched 24V lamps (tungsten, but not halogen types) with the proper wattage looks promising on 60V rails, also from life expectance (running at ~80% rated voltage -- but one still needs a healthy glow, otherwise filament notching at local hot spots will counteract). Sort of "power good" indicators as well.
Regards, Klaus
just an idea: lamps instead of the heavier bleeders. With lamps, at their working voltage, one can have at least 5 times the low voltage resistance. A stack of three matched 24V lamps (tungsten, but not halogen types) with the proper wattage looks promising on 60V rails, also from life expectance (running at ~80% rated voltage -- but one still needs a healthy glow, otherwise filament notching at local hot spots will counteract). Sort of "power good" indicators as well.
Regards, Klaus
Transformers
Plitron has some whopping 2KVA 120VCT transformers as surplus (check the surplus listings). At around $90 each they aren't free, but in terms of VA per dollar they look pretty good; a reasonable person could probably run a stereo Leach off just one of them. In a moment of delirium I bought two, seeing as how I couldn't stomach paying so freaking much to UPS to just ship one. This may possibly be overkill for a mono Superamp. Folks in Toronto can pick them up in person and avoid being Deliveranced by UPS.
I bought a set of parts and partially completed Superamp boards from a guy who'd given up (something about his wife wanting the kitchen table back). He'd bought some massive 120/120 to 120/120 isolation transformers, something like 1.5 kVA. I wasn't enthusiastic about using those, so the amp bits have been languishing in boxes for a very long time. Presumably I do have genuine parts since they were all sourced back in the '80s.
Plitron has some whopping 2KVA 120VCT transformers as surplus (check the surplus listings). At around $90 each they aren't free, but in terms of VA per dollar they look pretty good; a reasonable person could probably run a stereo Leach off just one of them. In a moment of delirium I bought two, seeing as how I couldn't stomach paying so freaking much to UPS to just ship one. This may possibly be overkill for a mono Superamp. Folks in Toronto can pick them up in person and avoid being Deliveranced by UPS.
I bought a set of parts and partially completed Superamp boards from a guy who'd given up (something about his wife wanting the kitchen table back). He'd bought some massive 120/120 to 120/120 isolation transformers, something like 1.5 kVA. I wasn't enthusiastic about using those, so the amp bits have been languishing in boxes for a very long time. Presumably I do have genuine parts since they were all sourced back in the '80s.
Problems with bass response
Hi everyone,
I've just finished building a Leach Amp project that's been going on for almost a year. It's been my first such a big electronics project and I wanted to do it with care, that's why the long building time. After some initial problems with humming that I was able to solve by modificating the wiring everything seemed to be fine and both channels are working, even the sound is excellent.
The only problem left is the bass response. After short listening I felt there might be some bass lacking (the loudspeakers are at the moment pretty small, thus you cannot ever expect any excellent bass). On yesterday, I tried the amp with bigger loudspeakers of a friend of mine and it became obvious that something is not OK.
As a next step we controlled the frequency response by means of a signal generator and an oscilloscope. Between approx. 250Hz and 20kHz (upper frequency limit of our signal generator) everything was fine, but below 250 Hz the response sank rapidly. At 50Hz the output level was well under the half of what it should be. Strangely, I have the the same problem on both channels with exactly similar behaviour. Probably some component I've used is thus of wrong type or just of lousy quality.
At first I suspected the capacitor C6 on feedback loop, because according to the documentation of the amp problems with this capacitor might lead to loss of bass response. The specs for C6 were 220u/16V bi-polar electrolytic, I have 220u/35V, because a 16V was not available. The jumper wire for holes of 6B is of course soldered, otherwise there wouldn't be any bass response of all. I tried to bridge C6 with a jumper wire and this didn't cause any major change on the sound so C6 seems to be an open loop for audio frequencies, like it should be.
Now I'm wondering what to do next. My first idea would anyway be to replace the bi-plar capacitor with two normal electrolytics as indicated possible in the documentation. Normal electrolytics of a good quality are easy to find, the bi-polars are much rarer and the one I've used is the only one I could find and I don't know if it good or not.
Before I go further I would like to know if there are some other potential causes for this type of problem and how they could be measured or tested otherwise. I wouldn't like to change any components for vain cause it's not very good for the circuit board to solder components off and on. Any help would be greatly appreciated
.
Hi everyone,
I've just finished building a Leach Amp project that's been going on for almost a year. It's been my first such a big electronics project and I wanted to do it with care, that's why the long building time. After some initial problems with humming that I was able to solve by modificating the wiring everything seemed to be fine and both channels are working, even the sound is excellent.
The only problem left is the bass response. After short listening I felt there might be some bass lacking (the loudspeakers are at the moment pretty small, thus you cannot ever expect any excellent bass). On yesterday, I tried the amp with bigger loudspeakers of a friend of mine and it became obvious that something is not OK.
As a next step we controlled the frequency response by means of a signal generator and an oscilloscope. Between approx. 250Hz and 20kHz (upper frequency limit of our signal generator) everything was fine, but below 250 Hz the response sank rapidly. At 50Hz the output level was well under the half of what it should be. Strangely, I have the the same problem on both channels with exactly similar behaviour. Probably some component I've used is thus of wrong type or just of lousy quality.
At first I suspected the capacitor C6 on feedback loop, because according to the documentation of the amp problems with this capacitor might lead to loss of bass response. The specs for C6 were 220u/16V bi-polar electrolytic, I have 220u/35V, because a 16V was not available. The jumper wire for holes of 6B is of course soldered, otherwise there wouldn't be any bass response of all. I tried to bridge C6 with a jumper wire and this didn't cause any major change on the sound so C6 seems to be an open loop for audio frequencies, like it should be.
Now I'm wondering what to do next. My first idea would anyway be to replace the bi-plar capacitor with two normal electrolytics as indicated possible in the documentation. Normal electrolytics of a good quality are easy to find, the bi-polars are much rarer and the one I've used is the only one I could find and I don't know if it good or not.
Before I go further I would like to know if there are some other potential causes for this type of problem and how they could be measured or tested otherwise. I wouldn't like to change any components for vain cause it's not very good for the circuit board to solder components off and on. Any help would be greatly appreciated
.
Re: re: Problems with bass response
Hi,
the circuit board and components have been built exactly according the original instructions, without any mods. Also the feedback loop is according to the picture in following link:
http://users.ece.gatech.edu/~mleach/lowtim/fdbk.html
The capacitor C6 I already mentioned, C8 is 180pF silver mica and C7 0,1u/63V (C7 cannot cause this kind of problems). Maybe I'll have to find a multimeter that can measure also capacitances and check that the C6 and C8 are according to the specs, I should have some spare capacitors from the same packages.
-Aleksi
JensRasmussen said:
Hi,
the circuit board and components have been built exactly according the original instructions, without any mods. Also the feedback loop is according to the picture in following link:
http://users.ece.gatech.edu/~mleach/lowtim/fdbk.html
The capacitor C6 I already mentioned, C8 is 180pF silver mica and C7 0,1u/63V (C7 cannot cause this kind of problems). Maybe I'll have to find a multimeter that can measure also capacitances and check that the C6 and C8 are according to the specs, I should have some spare capacitors from the same packages.
-Aleksi
Hi again,
it took some time until I got the work going on again, but now the bi-polar capacitor has been replaced by two ordinary capacitors in both channels and according to new measurements done today the amp is now performing flawlessly. On frequencies near 20kHz there is a just measurable sinking on the response, but it cannot without doubt be heard at all and further the response is now perfect down to at least 16 Hz, which is the lower limit of the frequency generator. Even response to test signal for TIM-distortion is fine.
The problem has thus now been solved and the amp is finally ready. Thank you all for help
.
it took some time until I got the work going on again, but now the bi-polar capacitor has been replaced by two ordinary capacitors in both channels and according to new measurements done today the amp is now performing flawlessly. On frequencies near 20kHz there is a just measurable sinking on the response, but it cannot without doubt be heard at all and further the response is now perfect down to at least 16 Hz, which is the lower limit of the frequency generator. Even response to test signal for TIM-distortion is fine.
The problem has thus now been solved and the amp is finally ready. Thank you all for help
.Re: Re: Leach amp problem
The Leach amplifier is a highly proven design that works well; most problems are straightforward and easily solved due to assembly/construction errors.
I'd be interested in hearing your ideas of good, newer designs. I've looked at quite a few but haven't found any compelling ideas to pursue other than Class A and Class D, which aren't very practical for my needs. (I've even got a partially assembled tube amplifier design, but that's been on hold for several years.)
Meanwhile, I keep coming back to the popular topolgy that Leach and many other amplifiers use, simply because it works.
guoming said:
The Leach amplifier is a highly proven design that works well; most problems are straightforward and easily solved due to assembly/construction errors.
I'd be interested in hearing your ideas of good, newer designs. I've looked at quite a few but haven't found any compelling ideas to pursue other than Class A and Class D, which aren't very practical for my needs. (I've even got a partially assembled tube amplifier design, but that's been on hold for several years.)
Meanwhile, I keep coming back to the popular topolgy that Leach and many other amplifiers use, simply because it works.
Hi, EMX. to answer your question , there are some poorly designed amp's with active CSS that may sound worse
than one without CSS.
It's all in the design.. I notice cheap consumer electronics
have the CSS free designs (compact stereo's), while
high end audio may have multiple current sources.
So , it's a matter of opinion , design and layout. If you want
to see an assortment of amp designs, click the bottom "WWW" of
my post or http://24.214.255.177/pdf1/Electronics/Projects/Audio_amp/
and see 200 amp designs ,download 'em all..
Leach is in there too..
Have fun.. OS
than one without CSS.
It's all in the design.. I notice cheap consumer electronics
have the CSS free designs (compact stereo's), while
high end audio may have multiple current sources.
So , it's a matter of opinion , design and layout. If you want
to see an assortment of amp designs, click the bottom "WWW" of
my post or http://24.214.255.177/pdf1/Electronics/Projects/Audio_amp/
and see 200 amp designs ,download 'em all..
Leach is in there too..
Have fun.. OS
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