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PR-800 ClassA/AB Amplifier Kit
PR-800 ClassA/AB Amplifier Kit
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Old 11th February 2016, 12:35 PM   #11
roclite is offline roclite  United Kingdom
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Join Date: Dec 2011
Location: Middlesex
PR-800 ClassA/AB Amplifier Kit
Default PR-800 Class A/AB

A first look at the PR-800 ClassA/AB design and layout revealed some interesting "features"

1) The IPS has MPSA42/2N5551 NPN Darlington input pair with a 2N5551 cascode in each collector arm.
2) In spite of 2 x 4700uF/rail PSU capacitors on-board next to the connection terminal block, there are no other localised decoupling electrolytics elsewhere.
3) There are no base stopper resistors on the output Toshiba power transistors.
4) No output Zobel network.
5) A large number of components are used in a SOA/fault current output protection circuit which has a latching SCR device, requiring a separate 6 - 15 VAC supply.

I have not come across a Darlington IPS before and there seems to be very little documentation about them - not a great start. I would be keen to remove/disable the protection circuit to avoid the need for a separate transformer or a low voltage winding on the mains transformer. As discussed, I will be using +/- 55 V DC supplies so the circuit is probably an overkill. The other things are easy to resolve by hacking a few pcb tracks and adding additional components but beginning to wonder if this is a good audio design to start with. Anyone out there with any relevant experience with this particular amplifier?
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Old 22nd February 2016, 11:45 PM   #12
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
Default PR-800 Class A/AB unwrapped

Before I started hacking the PR-800 boards, I thought it would be a good idea to sketch the circuit design from the board to review the design and map how the components were interconnected. At the same time, I learned to use LTspice as this would be useful to check the performance of the original PR-800 and any subsequent design changes I implemented. The performance "specifications" provided by the eBay seller listing were incomplete with no test conditions against any of the performance figures. I used LTspice to try simulate the Harmonic Distortion and Frequency response measurements against the claimed performance figures:

Harmonic Distortion: <0.01 10W/8 Ohm
Frequency Response: 1Hz - 160KHz (-3dB)

In my simulated amplifier, I set the supply rails to +/- 45 VDC and adjusted the input 1 KHz test signal to deliver 10W into 8 Ohm resistive load. I adjusted the output stage bias to the optimum class AB, approx. 120mA per transistor pair (4 parallel pairs) which should have given low distortion. The results screenshots are attached and the simulated performance figures:

Harmonic Distortion: <0.065% 10W/8 Ohm (10th order?) 10KHz bandwidth
Frequency Response: 1Hz - 500KHz (-3dB)

Unless I am using inaccurate component models or my understanding of LTspice is not up to scratch (very likely), these performance figures seem way off. If they are accurate, at least I know my starting point
Attached Images
File Type: png PR-800_freq_resp.png (80.1 KB, 573 views)
File Type: png PR-800_THD.png (48.3 KB, 534 views)

Last edited by roclite; 22nd February 2016 at 11:51 PM.
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Old 23rd February 2016, 09:14 AM   #13
Ian Finch is offline Ian Finch  Australia
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You show a thumbnail schematic used for simulation but it's illegible. Can you post the asc. simulation file or just the schematic so that there is a reference for the results?
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Old 23rd February 2016, 11:01 AM   #14
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
Hi Ian, that was deliberate because I didn't want to publish the circuit detail on-line. Though its origins are unknown and it may well have been cobbled together from other published designs, I observe the original designers copyright. Besides, I know if I publish the .asc simulation file on this forum, every man and his dog will "re-design" it for me thus denying me the opportunity to teach myself. With Cordell and D. Self amplifier volumes at the ready and the excellent information available on this forum, I should be able to tweak the design within the constraints of the existing layout and probably improve performance before I build into my amplifier case. Note, i have not done any actual testing of the amplifier performance and my LTspice simulation could well be wrong but its somewhat irrelevant since I will be hacking the existing circuit anyway.

As a result of investigating the circuit design, I discovered that the requirement for the additional 6 - 15 VAC supply was to provide separate DC power rail for a current source for the LTP current bias transistor, not the protection circuit as previously thought. So, there is clearly opportunity to change this and avoid the need for a separate transformer winding or transformer. The protection circuit is different in that it is not the usual dynamic V/I limiter but an SCR-triggered design which shuts off the LTP bias, presumably if an excessive current flows through the output stage first transistor pair. I assume you would reset the amplifier by switching OFF/ON if that condition was reached. Not sure if I want/need that kind of "protection" as its of limited use anyway. The other thing the LTspice simulation flagged up was that output DC offset was likely to be high so this could be minimised by matching transistor pairs or with some adjustment trimmer.

Last edited by roclite; 23rd February 2016 at 11:08 AM.
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Old 27th February 2016, 10:11 PM   #15
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
I managed to get up to speed with LTspice and spent more hours than was really healthy tweaking the original PR-800 Class A/AB design into something much better. I was very grateful to astx and other diyaudio members for their brilliant work on this thread:

http://www.diyaudio.com/forums/solid...8r-400w4r.html

which was basically a master-class tutorial in how to use LTspice to design, test and evaluate a class AB amplifier. This work was even more relevant to mine because the PR-800 circuit design was quite similar and could be modified fairly easily using the same configuration in the 2-stage class AB amplifier.

But first, I revisited the original PR-800 design to re-evaluate using the same DC supplies and input signal conditions as the simulated new design. The results were much worse than before because now it appeared that the PR-800 displayed asymmetrical clipping at +46/-34V levels on +/- 59V DC supplies, probably due to the protection circuit or bad biasing. I had not seen that during previous simulations at 10W into 8 ohm. It didn't look good, that protection circuit really had to go

Since the new design is now quite different from the original PR-800, I have no issues with sharing the LTspice files and component models for the new amplifier now renamed to SAB-800. I copied over as much of the 2-stage class AB design as I could bearing in mind that I was implementing this on an existing amplifier layout, so could not make it identical. Nevertheless, from the results, the THD figures are now orders of magnitude less than before and there is no evidence of premature asymmetrical clipping behaviour. The biggest reduction in simulated THD figures was when I copied over the Q9/Q12/Q13 VAS driver stage (Q8/Q9/Q25 on SAB-800). Of course, these are only LTspice simulated performance figures so actual implementation could be significantly different. There may be further component tweaks to improve performance even more but the issue here is that I will always be constrained by implementing the final circuit on the existing PR-800 amplifier board.
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File Type: png SAB-800.png (146.6 KB, 526 views)
Attached Files
File Type: zip SAB-800.zip (52.5 KB, 131 views)
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Old 28th February 2016, 07:30 AM   #16
AndrewT is offline AndrewT  Scotland
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Nice piece of investigative work.
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Old 28th February 2016, 11:45 AM   #17
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
Thanks Andrew, don't think I can take much credit for other members hard work but it does show that it might be possible to make a "silk purse out of a sow's ear". A definite thumbs-up for cut'n'paste audio design. All I need to do now is implement onto the PR-800 board. Luckily, by removing the dubious protection circuit, I have plenty of free PTH for the extra components and have also become very adept with my Dremel. It wont look pretty but you don't listen with your eyes, after all.
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Old 28th February 2016, 03:41 PM   #18
astx is offline astx  Austria
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PR-800 ClassA/AB Amplifier Kit
Dear roclite,

increase OPS bias to oliver point and you will get even better distortion figures. Currently you have biased one OPS bjt pair with about 25mA - increase it to e.g. 90mA and you will get better distortion figures (asc file: reduce R13 from 655R to 625R). If you want to keep lower BIAS current you have to increase the emitter resistors up to 0.47R.
Try to add "TianProbe()" parts to check the NFB loop stability.

Have fun, Toni
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Old 28th February 2016, 07:17 PM   #19
roclite is offline roclite  United Kingdom
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PR-800 ClassA/AB Amplifier Kit
Hi Toni

Thanks for your advice and many thanks for sharing your 2 Stage Class AB amplifier design, I learned a lot reading about the design steps you took during the development.

I keep the OPS bias quite low to reduce quiescent power dissipation in the output stage and driver transistors, mostly TO-92. The SAB-800 boards will be installed into an old amplifier case re-using switches, output terminals, heatsinks etc so usually the heatsinks are not optimum size. Replacing the 0.22R emitter resistors is likely to damage the boards so I would keep as is. Apart from that, there are 16 of them. I will try the Tianprobe before I implement the changes. I did notice what looks like a bit of frequency ripple/instability on the square-wave trailing edge that meant I had to increase my base stopper resistors from 4.7 to 15 Ohm, probably higher than I would like.
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Old 17th June 2016, 11:02 PM   #20
Mor is offline Mor
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Hey, you guys brings me hope!! :-D A Year ago i did buy 14 PR-800 Amplifiers to drive by mini-disco, but haves big problems with the PR-800 in the Subwoofers. Have 2 PR-800 in bridge for each driver, but it goes in protection-mode very often :-/ Not funny when you are on a job... Had tryed to fix the problem by putting a small-capasitor on each input and measured and changed the bridging circuit, but it seems it is the protection-part on the PR-800 Boards there just is to protective :-S On the final output i uses at external protection-board and it is fine. So i vill be so thankful if you can help me, or guide me to set the inbuilt protection circuit out of function! :-) (Sorry Im from Denmark not good to English) :-) Please help :-)
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