I have been uncomfortable with his designs for some time now since there have been reports of oscillations here. That is a subwoofer amp and it has no protection other than fuses as I understand it. It has an interesting CFP driver with the EF output stage ... might be interesting to try but I'm almost done with a redesign of this amp if it still interests you.
Yes, I'm still interested. Would it make sense to put two modern heat sinks inside for the transistors, in close proximity to the board with one on each side? This would eliminate the long wire bundle to the transistors and should help with stability. And I can still put a fan in if need be. I'm not opposed to etching a new board for your refinement of the original design either. I've done a lot of PCB layouts for various projects.
nah I thought the ADCOM comment would of got to you more. Yes I'm degree'd up, but worked my way thru school as a production tek before that, That's exactly why I can sling the mud ie no sacred cows here. I have an extensive design background and been around the block a few times in different industries, doesn't bother me at all. BTW I've built this 70W RCA design with my own PCB in HS (RCA 1974 data book page 602). I've built and tested the AA Borbley FET PA as well. also repaired quite a few receivers. Some Marantz, Harmon Kardon (uses RCA topology), bunch of other Jap stuff from 200W monsters to flea powered receivers. I pretty much already said what my approach to get this working again. I think the VAS and AB bias needs some rework. I'd get it working first with a single set of outputs. Probably most critical is the driver wiring and relocating some PCB parts 2200pF at the driver leads. Then I'd test for stability and any compensation adjustments. If it was way off I'd drop the design. If not I would speed up the protection circuit looking at the data book for ideas substituting larger values of RE to test.. If I ever get a big surplus PS here I would beef up the RCA drivers and outputs using some surplus 2N6259 as outputs, these have the most SOA of any devices I know.
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You can buy the Raw PCBs and order the rest at Mouser or Digikey.
300/500W Subwoofer Power Amplifier
300/500W Subwoofer Power Amplifier
OK infinia show us your marked up schematic with all your revisions!
Most of the suggested amps here are for HiFi, note Rod's mention of his guitar amp being overbuilt, and that it has output protection, yet he says it still might blow outputs with a short - hmm not very good protection:
100W Guitar Amplifier (Mk II)
It shows that VI limiting is needed since the short circuit current limiting should be lower than that with a 2 or higher ohm load.
Leach shows a VI circuit here:
Leach Amp Protection Circuit
His D9 and D10 protect the protection transistor from excessive reverse bias which I noted earlier as a flaw in this design. Such diodes should be added to this design.
Protection discussion by Self see page 454:
Audio Power Amplifier Design Handbook - Douglas Self - Google Books
The protection transistors should be mounted to the heat sink to lower the current limit at high temp,
note the Vbe on vs. temp curve here; seems 2n4401/4403 would also make good protection transistors
with their fairly low Vbe and Vce sat voltages:
Semiconductor and Integrated Circuit Devices
Most of the suggested amps here are for HiFi, note Rod's mention of his guitar amp being overbuilt, and that it has output protection, yet he says it still might blow outputs with a short - hmm not very good protection:
100W Guitar Amplifier (Mk II)
It shows that VI limiting is needed since the short circuit current limiting should be lower than that with a 2 or higher ohm load.
Leach shows a VI circuit here:
Leach Amp Protection Circuit
His D9 and D10 protect the protection transistor from excessive reverse bias which I noted earlier as a flaw in this design. Such diodes should be added to this design.
Protection discussion by Self see page 454:
Audio Power Amplifier Design Handbook - Douglas Self - Google Books
The protection transistors should be mounted to the heat sink to lower the current limit at high temp,
note the Vbe on vs. temp curve here; seems 2n4401/4403 would also make good protection transistors
with their fairly low Vbe and Vce sat voltages:
Semiconductor and Integrated Circuit Devices
Why? I got nothing to prove on this, unlike some folks here? Wouldn't it be a waste of my "valuable internet time" esp since the OP decided against my outlined approach. LOL
Besides, I thought we were moving forward already. I seconded the OPs motion to use the ESP PCB. Now you are forcing the OP and myself, what you want? Demanding will get you everywhere, NOT.
Now if the OP wants to stick with the original design and asks me for help that would be a wholly different story. I can guarantee that I could get that thing to work better than new with mostly wiring changes and a handful of parts changes, at the bench with test gear. I esp don't want to get in a willy waving exercise cause of your demands. Above all your demostrated hostile behavior toward my posts ie nitpicking words and ignoring all positive technical points of my previous contributions in this thread.
Infinia, Pete indicated a few posts back that he was working thru a redesign of the current amp and I indicated that yes, I am interested. If you have suggested mods to make the current layout work I'd be interested in seeing what you come up with as well. As I mentioned before, I'm new to solid state power amps so I'm going to have to learn as I go with this one and I'm not going to able to take general theories and ideas and develop them into implimented circuits without some help. That being said, I do have a lot of overall electronics experience with tube amps and solid state signal level stuff. I've built around 30 tube amps, many of my own design, and I'm not afraid to tackle a project that pushes me to learn more so long as it doesn't become a major lab project.
One thing I see that would be a big improvement would be to add real heat sinks and I'm not afraid to modify the chassis to do that. If the current PCB is still retained, then dual heat sinks would probably work best with one on each side of the PCB but if you have other ideas, I'm willing to listen.
Even if the redesign retains much of the design elements of the original amp design, I'm not opposed to etching a new PCB, especially if it makes it easier to impliment a specific heat sink arrangement.
In the end, I want a reliable amp the handles the rigors of bass guitar, which includes the sometimes not so bright bass player that forgets to hook up or shorts the speaker connections. I also don't want to spend the family fortune to accomplish this.
One thing I see that would be a big improvement would be to add real heat sinks and I'm not afraid to modify the chassis to do that. If the current PCB is still retained, then dual heat sinks would probably work best with one on each side of the PCB but if you have other ideas, I'm willing to listen.
Even if the redesign retains much of the design elements of the original amp design, I'm not opposed to etching a new PCB, especially if it makes it easier to impliment a specific heat sink arrangement.
In the end, I want a reliable amp the handles the rigors of bass guitar, which includes the sometimes not so bright bass player that forgets to hook up or shorts the speaker connections. I also don't want to spend the family fortune to accomplish this.
In addition to other suggested solutions, I've been looking at the Leach amp that was mentioned in an earlier post and find it quite interesting. Many cool features to that amp. Not necessarily easy to implement with my current PCB but might be a good way to get this up and running and it fits my power transformer well being designed specifically for 56V rails.
Do any think that there should be mods done to the Leach circuit knowing it will be for bass guitar use?
It's probably been infered but I'm planning on using the preamp section of this amp without mods. Well, except that I am going to replace ALL the electrolytic caps. The preamp is the main influence to the overall character of this amp anyway and that will definitely be retained.
Do any think that there should be mods done to the Leach circuit knowing it will be for bass guitar use?
It's probably been infered but I'm planning on using the preamp section of this amp without mods. Well, except that I am going to replace ALL the electrolytic caps. The preamp is the main influence to the overall character of this amp anyway and that will definitely be retained.
Did you happen to come across these surplus heat sinks? It's been a while since I've bought any, don't know how the price compares to new or how they might fit in the enclosure: LARGE HEATSINK FOR TWO TO-3 DEVICES | AllElectronics.com
The Leach design is good but a bit dated, with low gain outputs and a triple EF output stage. It is fully comp from input to output which should roughly cancel even distortion, but this is the type that you want since it is less harsh than odd. I don't think he rated it for 2 ohm loads but you could always add 1-2 more pairs of outputs. I would build it with more modern output devices and I think there are many threads here covering Leach updates. I do not follow them so.... You'll also find boards here for updated Leach designs.
You could use the current board to do a revised version of the original design; I doubt that you will have to hack it up much at all. Then when the updates are working well you could do a new board design if you wanted to or you could use Rod Elliot's guitar amp board for a revised version of this design since it is so similar.
If you want to get the most output into two ohm loads and you want it to be more rugged 3-4 pairs of outputs make a lot of sense and they could be plastic types to save a few bucks.
The Leach design with newer components is not a bad choice just a bit more complicated.
The Leach design is good but a bit dated, with low gain outputs and a triple EF output stage. It is fully comp from input to output which should roughly cancel even distortion, but this is the type that you want since it is less harsh than odd. I don't think he rated it for 2 ohm loads but you could always add 1-2 more pairs of outputs. I would build it with more modern output devices and I think there are many threads here covering Leach updates. I do not follow them so.... You'll also find boards here for updated Leach designs.
You could use the current board to do a revised version of the original design; I doubt that you will have to hack it up much at all. Then when the updates are working well you could do a new board design if you wanted to or you could use Rod Elliot's guitar amp board for a revised version of this design since it is so similar.
If you want to get the most output into two ohm loads and you want it to be more rugged 3-4 pairs of outputs make a lot of sense and they could be plastic types to save a few bucks.
The Leach design with newer components is not a bad choice just a bit more complicated.
I have always preferred convection for HiFi amps since we want them to be quiet, however fans have a huge advantage, even the hole size of the screen matters when you rely on convection. Then if you stack the equipment, or someone throws a stack of papers on top, fans are just much better.
I have a lot of 120V fans and can send you one if you'd like but you might want to do something clever with DC PC computer fans; such as running them very slow at a whisper speed normally and kick them up when it gets hot. I got ten 12VDC PC fans for $12 from NewEgg.
A trick they do with PCs is to have the fans pull outside air in so that you get the outside cooler air running over the heatsinks first, PCs can easily run 10 degrees hotter inside than the outside ambient.
I have a lot of 120V fans and can send you one if you'd like but you might want to do something clever with DC PC computer fans; such as running them very slow at a whisper speed normally and kick them up when it gets hot. I got ten 12VDC PC fans for $12 from NewEgg.
A trick they do with PCs is to have the fans pull outside air in so that you get the outside cooler air running over the heatsinks first, PCs can easily run 10 degrees hotter inside than the outside ambient.
I like the approach of using the existing board if it doesn't take much hacking to accomplish. The heat sinks you mentioned look like a match with that also. I could even mount them on the back of the chassis out in the open but then my leads get a little longer. Speaking of that, Just how long is too long. Obviously, the original design had very long leads to the transistors but what should I aim for?
I will never run the amp at 2 Ohm load, I'll often run it at 4 Ohm but rarely at full power when I do. I would like to stay with a quad of output transistors if possible.
Outside is good, you might be sorry after getting poked by them a few times. Inside is fine also just use a grill with good size holes. Can you just relocate the driver board to keep the leads down to 3 or 4 inches? I've added base stoppers to the design and that should help with oscillations, the EF output stage is less prone to oscillations also.
You should be fine with 2 pairs of large devices and competently designed output protection with 4 ohm loads. In fact you can just use the Leach output protection component values since it operates on the same supply voltage and has 2 pair of similarly rated output devices. His emitter resistors are .33, yours .27, I'd just change R37,39,38,40 back to his old value of 470 ohms to compensate for the Re difference - and then I'd also test it to confirm a good limit point. Remember I commented on the 22 uF in this design, Leach uses .1 uF to slow down the protection.
Rod's amp looks a lot like this:
http://www.rickenbacker.com/pdfs/19333.pdf
From:
Solid State Amp Schematics
This might be of interest:
http://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf
Thanks Pete, Teemu's book is already helping me. I'm trying to get an overall better understanding of the building blocks of this circuit. Since I come from tube amp background, I have some old thought habits I need to break. Anytime I see a differential pair, I think it's a phase inverter. That's what was confusing me. Now I see better what's happening with Q8 and Q12 also.
Is the differential pair only so there is an easy way to inject the feedback? When I build guitar amps, I often push the feedback in at the differential pair of triodes but they provide a phase inverter function as well.
I'm also having a bit of trouble understanding how the input gets to Q1, is it that R2 and R3 mirror the VAS? so when the voltage between the Q8 and the 56V rail changes, R2, R3 just follows in a opposite manner, so the VAS is becoming the phase inverter? If this is the case, then it seems to me that at idle, the voltage between the rail and the Q2 input would be 56V minus half the bias diode drop and then what ever voltage swing comes out of the VAS would lay on top of this starting voltage.
Also, is C4 providing some local FB?
I know this is all second nature to many of you but my old brain is trying to wrap around all this. Thanks for your patience.
Thanks for giving me the tools to make me think and understand what's happening. I guess a structures engineer CAN learn electronics.
Is the differential pair only so there is an easy way to inject the feedback? When I build guitar amps, I often push the feedback in at the differential pair of triodes but they provide a phase inverter function as well.
I'm also having a bit of trouble understanding how the input gets to Q1, is it that R2 and R3 mirror the VAS? so when the voltage between the Q8 and the 56V rail changes, R2, R3 just follows in a opposite manner, so the VAS is becoming the phase inverter? If this is the case, then it seems to me that at idle, the voltage between the rail and the Q2 input would be 56V minus half the bias diode drop and then what ever voltage swing comes out of the VAS would lay on top of this starting voltage.
Also, is C4 providing some local FB?
I know this is all second nature to many of you but my old brain is trying to wrap around all this. Thanks for your patience.
Thanks for giving me the tools to make me think and understand what's happening. I guess a structures engineer CAN learn electronics.
Hi Carl,
I really wish there was an in depth description of the Leach amp and the evolution, I think that would be a good discussion. I have not read this most recent document in depth so I can't say how accurate it is; at least it has been reviewed.
Diff pairs have much better linearity than a singled ended device and in an amp it is a natural at the front end to provice DC balancing of the output, yes, through the negative feedback. Power amps certainly have been built without them but they are rare. This diff amp would provide a balanced output if there was a 1K resistor in the collector lead of Q5, and the resistor is left out since it is not needed.
I think some basics might help here in your understanding, transistors have to be in the forward bias region in amplifier applications. The base emitter junction should be forward biased with .5 to .8V, since it is like a diode, depending on how much base current is provided. The B-E junction will take a good amount of current and still only drop 1 to 2V until it is destroyed. Next, the output stage is a common collector configuration, in our reworked design, AKA emitter follower which is similar to a cathode follower having nearly unity non-inverting gain. This tells us that the top of the bias network (DD1) for the output stage is going to be at about 2 diode drops or about +1.4V, and the bottom of the bias network will be at about -1.4V. We can calculate the VAS quiescient or idle current also knowing that the base of the emitter followers draws very little current such that for this approximation it can be ignored. Now calculating the VAS idle current is simple = (56 - 1.4) /(2*1800) = 15.2 mA. This is high but Q8 is a slow device with a lot of capacitance to drive and the output stage is not a triple so it will need a good amount of drive current.
To answer your question R2 and R3 are the VAS collector load with DD1 just providing a few diode drops to bias the output stage. R2 and R3 combined with C4 provide an interesting trick known as a bootstrap to approximate a current source load to the VAS, and to provide a boosted voltage source. I have a post explaining this in more detail. It significantly increases the voltage gain of the VAS:
Patchwork Reloaded: Circuit Optimization and Board Layout. - Page 11 - diyAudio
This was written regarding this schematic which is flipped relative to the Traynor:
Patchwork Reloaded: Circuit Optimization and Board Layout. - Page 11 - diyAudio
Keep in mind that as the output stage drives high current into the load with signal swing more base drive is needed from the drivers, and thus from the VAS to the drivers.
I would really like to see schematics for the Rickenbacker Transonics amps if anybody has them. The TR amps on the Rickenbacker web site are not the same as I understand it.
I really wish there was an in depth description of the Leach amp and the evolution, I think that would be a good discussion. I have not read this most recent document in depth so I can't say how accurate it is; at least it has been reviewed.
Diff pairs have much better linearity than a singled ended device and in an amp it is a natural at the front end to provice DC balancing of the output, yes, through the negative feedback. Power amps certainly have been built without them but they are rare. This diff amp would provide a balanced output if there was a 1K resistor in the collector lead of Q5, and the resistor is left out since it is not needed.
I think some basics might help here in your understanding, transistors have to be in the forward bias region in amplifier applications. The base emitter junction should be forward biased with .5 to .8V, since it is like a diode, depending on how much base current is provided. The B-E junction will take a good amount of current and still only drop 1 to 2V until it is destroyed. Next, the output stage is a common collector configuration, in our reworked design, AKA emitter follower which is similar to a cathode follower having nearly unity non-inverting gain. This tells us that the top of the bias network (DD1) for the output stage is going to be at about 2 diode drops or about +1.4V, and the bottom of the bias network will be at about -1.4V. We can calculate the VAS quiescient or idle current also knowing that the base of the emitter followers draws very little current such that for this approximation it can be ignored. Now calculating the VAS idle current is simple = (56 - 1.4) /(2*1800) = 15.2 mA. This is high but Q8 is a slow device with a lot of capacitance to drive and the output stage is not a triple so it will need a good amount of drive current.
To answer your question R2 and R3 are the VAS collector load with DD1 just providing a few diode drops to bias the output stage. R2 and R3 combined with C4 provide an interesting trick known as a bootstrap to approximate a current source load to the VAS, and to provide a boosted voltage source. I have a post explaining this in more detail. It significantly increases the voltage gain of the VAS:
Patchwork Reloaded: Circuit Optimization and Board Layout. - Page 11 - diyAudio
This was written regarding this schematic which is flipped relative to the Traynor:
Patchwork Reloaded: Circuit Optimization and Board Layout. - Page 11 - diyAudio
Keep in mind that as the output stage drives high current into the load with signal swing more base drive is needed from the drivers, and thus from the VAS to the drivers.
I would really like to see schematics for the Rickenbacker Transonics amps if anybody has them. The TR amps on the Rickenbacker web site are not the same as I understand it.
The "Leach amp" in its first iterations is not correctly designed.
It would be far better and easier to improve the existing circuit
of your amp.
Implementing a complementary output stage is quite easy,
not counting that there s some minor tweaks that can be made
to greatly improve the performances.
All in all, there wouldn t be more components count.
It would be far better and easier to improve the existing circuit
of your amp.
Implementing a complementary output stage is quite easy,
not counting that there s some minor tweaks that can be made
to greatly improve the performances.
All in all, there wouldn t be more components count.
Very cool, I see it now. I've used bootstrapping before with split load phase inverters using triodes with the in phase signal fed in between the dual load resistors of the preceding triode stage but when you do that you have to adjust the anode load resistor of the phase inverter to adjust for the change in output impedance to keep both sides of the phase inverter balanced. I've also used bootstrapping with JFET's to bump their gain up in phantom powered mic circuits.
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