Hello,
I'm happy to say that I've completed one channel of my OnSemi ThermalTrak amplifier. It is essentially a modified Leach clone (many thanks to Jens).
The amplifier is completely modular with onboard PSU and protection. Just plug in the transformer, signal input and speaker outputs. It measures 13.3" X 2.95" - the goal was to make it a compact module to enable me to build a 5-channel amplifier.
Picture:
(sorry, both the camera and my camera skills suck)
I'm happy to say that I've completed one channel of my OnSemi ThermalTrak amplifier. It is essentially a modified Leach clone (many thanks to Jens).
The amplifier is completely modular with onboard PSU and protection. Just plug in the transformer, signal input and speaker outputs. It measures 13.3" X 2.95" - the goal was to make it a compact module to enable me to build a 5-channel amplifier.
Picture:
(sorry, both the camera and my camera skills suck)
An externally hosted image should be here but it was not working when we last tested it.
I must add that I haven't been able to fully characterize electrically the Vbias circuit based on the ThermalTrak diodes. It works but so far, I can't tell if it is doing a better job than a plain old VBE amplifier transistor.
I have access to a thermal chamber for testing electronic products but I'm not sure if heating up the entire module and measuring Vbias proves anything. It's the junction temp vs Vbias that really interests me.
Other than that, the module's was completed yesterday and it sounds very good. Can't say too much as it is hard to compare a mono module vs my stereo hifi setup. Work is going slow on the 2nd module...
I have access to a thermal chamber for testing electronic products but I'm not sure if heating up the entire module and measuring Vbias proves anything. It's the junction temp vs Vbias that really interests me.
Other than that, the module's was completed yesterday and it sounds very good. Can't say too much as it is hard to compare a mono module vs my stereo hifi setup. Work is going slow on the 2nd module...
Hi Arius,
Why not just measure idling current before and after giving the amplifier a 'good exercise'? If the idling current dominates over the rest of the amplifier, that should be a pretty good test. Most amplifiers drift all over the place, specially from startup when everythng's cold...
Cheers!!
Why not just measure idling current before and after giving the amplifier a 'good exercise'? If the idling current dominates over the rest of the amplifier, that should be a pretty good test. Most amplifiers drift all over the place, specially from startup when everythng's cold...
Cheers!!
Hello Arius,
That's a really fine lookin' layout. Maybe you would be willing to tell more about the circuit and construction details. Like the purpose of a relay on the board and other small details.
How does the bias adjustment compare to a Leach board and so on.
Inquiring minds want to know.
Regards
That's a really fine lookin' layout. Maybe you would be willing to tell more about the circuit and construction details. Like the purpose of a relay on the board and other small details.
How does the bias adjustment compare to a Leach board and so on.
Inquiring minds want to know.
Regards
Hello Ace,
Thanks for your inquiry. Um, I did post the schematics here so all the details are there.
The relay is for DC fault, turn on/off, overtemp protection. Bias is adjusted just like a regular Leach or any other regular Emitter-Follower class b amp.
There's really nothing special about this amp other than the ThermalTrak's diodes reacting faster to temp changes. Like Clem suggested, I think I'll use my test setup and 300W dummy load to run the module through various levels of input signal and see how the Vbias tracks temperature changes. It won't be accurate as I'll be measuring Vin vs Vbias instead of Tjunction.
BTW, my toroidal transformer is Antek Inc's 700VA type. It has pretty good load regulation. Rail voltage dropped from 53V to 50V at full load. With 50V rails, I get Vout = 44V before clipping so that's 120W into 8R. Heatsink is a Conrad MF25-75.
The PCB is double sided with ground planes. Noise/hum is inaudible even with the input floating. The purists probably won't like to see the PSU circuit onboard, but my goal is for a compact, modular amp to enable 5 channels - minus the wiring nightmare. To remove one module from the chassis, there are only 3 connectors to unplug (4 if one chooses to bring the status led to the front panel) and 2 screws.
If anyone wants any detail, just ask away. Just keep in mind I'm not an amplifier expert. Just know a bit here and there. I'll start work on the AC soft-start/soft poweron board next.
Thanks for your inquiry. Um, I did post the schematics here so all the details are there.
The relay is for DC fault, turn on/off, overtemp protection. Bias is adjusted just like a regular Leach or any other regular Emitter-Follower class b amp.
There's really nothing special about this amp other than the ThermalTrak's diodes reacting faster to temp changes. Like Clem suggested, I think I'll use my test setup and 300W dummy load to run the module through various levels of input signal and see how the Vbias tracks temperature changes. It won't be accurate as I'll be measuring Vin vs Vbias instead of Tjunction.
BTW, my toroidal transformer is Antek Inc's 700VA type. It has pretty good load regulation. Rail voltage dropped from 53V to 50V at full load. With 50V rails, I get Vout = 44V before clipping so that's 120W into 8R. Heatsink is a Conrad MF25-75.
The PCB is double sided with ground planes. Noise/hum is inaudible even with the input floating. The purists probably won't like to see the PSU circuit onboard, but my goal is for a compact, modular amp to enable 5 channels - minus the wiring nightmare. To remove one module from the chassis, there are only 3 connectors to unplug (4 if one chooses to bring the status led to the front panel) and 2 screws.
If anyone wants any detail, just ask away. Just keep in mind I'm not an amplifier expert. Just know a bit here and there. I'll start work on the AC soft-start/soft poweron board next.
That is a very good looking getup you have there. I am impressed and I hope to see more commentary and especially other members trying it for themselves. I have always been a fan of the Leach amplifier, I wonder how the sound of this compares...?
Anyway, I like the modular concept and you get 0 points deducted for taking a long time making the 2nd channel, we all know how that goes.
Anyway, I like the modular concept and you get 0 points deducted for taking a long time making the 2nd channel, we all know how that goes.
acenovelty said:
It is probably because then the bias circuit would have gotten too high voltage = too much bias.
Each diode will add like 0.6-0.7 Volt.
The idle current bias in output is set by the voltage between Q5 BASE and Q25 BASE.
Depending how you design this bias circuit and type of output stage
you may only need to use 1-2 diodes in some cases.
(if you only have one output pair, you will only have 2 diodes)
Those two free diodes can be used for other thing.
For example a LED indicating a certain temperature,
either that amplifier has reached working temp at switch on
or a red warning LED for overheat.
Are these imbedded diodes different from the 1N4004's on the Leach LowTim amp? That amp and the Superamp use only 4 diodes. Prof. Leach mentions that 4 only made it convenient for them to come out on the same side of the heat sink.
Here Arius has used 3 diodes per side(6 total) and 4 transistors per side(8 total). Just trying to make sense of the logic or math involved.
The amp looks very impressive and well executed.
Regards
Here Arius has used 3 diodes per side(6 total) and 4 transistors per side(8 total). Just trying to make sense of the logic or math involved.
The amp looks very impressive and well executed.
Regards
Lineup is correct about me not using all 8 of the ThermalTrak diodes. Reasons:
1) 6 diodes is good enough to set Vbias accurately. Simulations confirmed this. It did not overcompensate (I think).
2) 6 diodes is what OnSemi had used in their appnote. So I could bypass the VBE transistor and just used the 6 diodes in a string to set Vbias - no adjustment.
3) I did not want to be forced to use 4 pairs of output devices all the time. 3 pairs is sufficient to deliver 300W into 2ohms. The 4th pair was added at my friend's (band player) insistence.
I regarded the embedded diodes as equivalent in function to the 1N4004's in the Leach amp.
The last two embedded diodes are unused. The 4th pair may be dropped (ThermalTrak's are cheap at OnSemi but pricey at Digikey).
There is onboard thermal cutoff and the status led will flash if that occurs. I thought of using one of the last diodes for temp sensing but I didn't want to deal with an opamp and its power supply hassle.
I'm still sniffing around for a spectrum analyzer. I'll also post more about sound quality when the 2nd module is finished so I can listen to a stereo setup.
One last thing:
Digikey = $7.43
Future = $6.00
OnSemi = $1.85
Being a "frugalphile", I only embarked on this because OnSemi was willing to send me samples although I told them outright it was for a personal project. Don't see how they're gonna sell many of them via Digikey. Hmph!
1) 6 diodes is good enough to set Vbias accurately. Simulations confirmed this. It did not overcompensate (I think).
2) 6 diodes is what OnSemi had used in their appnote. So I could bypass the VBE transistor and just used the 6 diodes in a string to set Vbias - no adjustment.
3) I did not want to be forced to use 4 pairs of output devices all the time. 3 pairs is sufficient to deliver 300W into 2ohms. The 4th pair was added at my friend's (band player) insistence.
I regarded the embedded diodes as equivalent in function to the 1N4004's in the Leach amp.
The last two embedded diodes are unused. The 4th pair may be dropped (ThermalTrak's are cheap at OnSemi but pricey at Digikey).
There is onboard thermal cutoff and the status led will flash if that occurs. I thought of using one of the last diodes for temp sensing but I didn't want to deal with an opamp and its power supply hassle.
I'm still sniffing around for a spectrum analyzer. I'll also post more about sound quality when the 2nd module is finished so I can listen to a stereo setup.
One last thing:
Digikey = $7.43
Future = $6.00
OnSemi = $1.85
Being a "frugalphile", I only embarked on this because OnSemi was willing to send me samples although I told them outright it was for a personal project. Don't see how they're gonna sell many of them via Digikey. Hmph!
Arius said:
I found this OnSemi PDF Application:
Single Channel Audio Amplifier with 3 pairs of ThermalTrak
MJL3281A / MJL1302A driven by
MJE15032 / MJE15033 and predrives
MPSA06 / MPSA56
and a differential NPN input stage.
Power supply is separate +-55 Volt for Input/VAS and +-45 Volt for Power output stage.
This will naturally improve performance.
http://www.onsemi.com/pub/Collateral/BRD8043-D.PDF
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Looks nice 
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