Hi Guys.
I have been working on the Leach Superamp that I built years ago but never got working. Through the help of some kind folks here on this thread, I was able to get it working, but I have a strange feedback sound when it has a load on it. I did a short video so you can see and hear what is happening. You can see that here.
http://www.youtube.com/watch?v=UYSs_mf9slY&feature=youtu.be
Those who have been helping me have not been able to determine what is causing it and so I am hoping with new eyes, someone may recognize it and give me some things to look for. Only one channel is doing this and it only does it with a load on the output, either a speaker or my 8ohm dummy load.
Thanks, Terry
I have been working on the Leach Superamp that I built years ago but never got working. Through the help of some kind folks here on this thread, I was able to get it working, but I have a strange feedback sound when it has a load on it. I did a short video so you can see and hear what is happening. You can see that here.
http://www.youtube.com/watch?v=UYSs_mf9slY&feature=youtu.be
Those who have been helping me have not been able to determine what is causing it and so I am hoping with new eyes, someone may recognize it and give me some things to look for. Only one channel is doing this and it only does it with a load on the output, either a speaker or my 8ohm dummy load.
Thanks, Terry
This is one of those things where the process of elimination is helpful.
One possibility is that the SOA limiting circuits are causing trouble. The easiest way to eliminate this possibility is to remove D7 and D8 (I assume that your component numbering is per Leach's web-site schematic).
There are other possibilities, but that is a good one to eliminate quickly. Just be careful not to short the output!
One possibility is that the SOA limiting circuits are causing trouble. The easiest way to eliminate this possibility is to remove D7 and D8 (I assume that your component numbering is per Leach's web-site schematic).
There are other possibilities, but that is a good one to eliminate quickly. Just be careful not to short the output!
OK, good call, that eliminated it. Now to find which device is causing it. I will start with replacing the active devices until I find it. Any suggestions on which ones to start with?
Thanks, Terry
Thanks, Terry
I'd start by checking resistors R28,29,37-40 and the power transistor emitter resistors (R45 to 48)
I agree completely with the previous post's suggestions.
I'd add to the list to check the polarity of D6.
I'd add to the list to check the polarity of D6.
So you guys are more thinking this is a passive component issue more so than an active component?
Thanks
Thanks
Don't know. But when current is drawn from the amp a voltage forms across the power transistor emitter resistors. This then gets divided down and the resulting voltage switches Q10 and Q11. If this voltage is wrong then you get the SOA protection kicking in too early.
This is a very simplified view of what is going on.
This is a very simplified view of what is going on.
So are you saying that too much DC offset can cause this? I do have a lot more DC offset on this board than the other. The good board has about 7mv offset and this one has 60mv.
I did find that R28&27 were wrong. I had stuffed 200R in there. That is the value for the LowTIM. The Superamp calls for 270R. Unfortunately, fixing that didn't solve the problem. If DC offset is causing this then I have a way to go yet.
Thanks
Another test...
The 60 mV offset is a bit high, but not crazily so...
So the next test might be:
With no signal input, measure and compare the voltages across R28 and R29.
Bkgnd...Q11 and SOA loop can only cause oscillation if Q11 turns on. It probably shouldn't be turning on...so, that's what the volts across R28 and R29 will give us a hint about...
Repeat the test with the load disconnected.
The 60 mV offset is a bit high, but not crazily so...
So the next test might be:
With no signal input, measure and compare the voltages across R28 and R29.
Bkgnd...Q11 and SOA loop can only cause oscillation if Q11 turns on. It probably shouldn't be turning on...so, that's what the volts across R28 and R29 will give us a hint about...
Repeat the test with the load disconnected.
Voltage across R28=0.00mv and across R29=16mv. That is with input shorted, with or without load attached. I measured the good board and it is R28=0.00mv and R29=9mv
A good hint...now we have to find where the voltage across R29 is coming from.
There are 3 possibilities...R31, R38, or R40. Measure the DC voltages across them, as well as DC voltages across R46 and R48.
Do the same measurements for R30, R37, R39, R45, and R47.
Make a table...
That should bring us closer...
There are 3 possibilities...R31, R38, or R40. Measure the DC voltages across them, as well as DC voltages across R46 and R48.
Do the same measurements for R30, R37, R39, R45, and R47.
Make a table...
That should bring us closer...
R31=0mv
R38=0mv
R40=0mv
R30=0mv
R37=0.6mv
R39=0.8mv
R45=20mv
R47=18.4mv
R46=19mv
R48=18.4mv
These are voltages across above resistors.
EDIT: D7,D8 have been reconnected. I should also mention that on the board, D5 and R30 are switched. D6 and R31 as well.
R38=0mv
R40=0mv
R30=0mv
R37=0.6mv
R39=0.8mv
R45=20mv
R47=18.4mv
R46=19mv
R48=18.4mv
These are voltages across above resistors.
EDIT: D7,D8 have been reconnected. I should also mention that on the board, D5 and R30 are switched. D6 and R31 as well.
Last edited:
Reported voltages aren't consistent...could be something is oscillating and disturbing the measurement, could be have you weren't really on R28.
R46 thru R48 have about 20 mV across each one.
R37-40 are 470 Ohms.
R28 and R29 are 200 Ohms.
Given no current in R30 and R31, then both R28 and R29 should have:
20 mV x (200/(200+470/2)=9.19 mV across them.
Yet you report R28 has 0 and R29 has 9 mV...
R29 value checks with theory...R28 doesn't...hmmm...why?
most likely...re-measure voltage across R28???
no conclusions yet..
R46 thru R48 have about 20 mV across each one.
R37-40 are 470 Ohms.
R28 and R29 are 200 Ohms.
Given no current in R30 and R31, then both R28 and R29 should have:
20 mV x (200/(200+470/2)=9.19 mV across them.
Yet you report R28 has 0 and R29 has 9 mV...
R29 value checks with theory...R28 doesn't...hmmm...why?
most likely...re-measure voltage across R28???
no conclusions yet..
Given the switches that you mention, can you be sure of the correct polarity for D5 and D6?
D5 and D6 are correct.
Note, on the Superamp R28 and R29 are 270R.
I just carefully remeasured. the amp has been running for about an hour now.
R28=0MV
R29=20.2MV
R30=0
R31=0
R37=0.8MV
R38=.1MV
R39=0.7MV
R40=0
R45=20.8MV
R46=19.7MV
R47=19.2MV
R48=19.9
DC offset = +64VDC
Note, If I measure across R30 and D5 is get 84.6mv
Across R32 and D6 I get 44.2mv
EDIT:
I just checked and I had reversed the amps. D28 and 29 should be 200R in the Superamp. That is what I had in there this morning. I will have to change them back. I will remeasure when I get them changed back.
Note, on the Superamp R28 and R29 are 270R.
I just carefully remeasured. the amp has been running for about an hour now.
R28=0MV
R29=20.2MV
R30=0
R31=0
R37=0.8MV
R38=.1MV
R39=0.7MV
R40=0
R45=20.8MV
R46=19.7MV
R47=19.2MV
R48=19.9
DC offset = +64VDC
Note, If I measure across R30 and D5 is get 84.6mv
Across R32 and D6 I get 44.2mv
EDIT:
I just checked and I had reversed the amps. D28 and 29 should be 200R in the Superamp. That is what I had in there this morning. I will have to change them back. I will remeasure when I get them changed back.
Last edited:
- Status
- Not open for further replies.