Resprom EMS 1511 Professional Rack Amplifier - from Bulgaria!

djduck

Active Member
Picked up this interesting piece today for €40, or about 50 American.
It's a professional rackmount power amplifier made in Bulgaria, in the early 80's, by a company called Resprom. Model EMS 1511. Part of the EMS 1000 set. Dual mono, 100w per channel. Originally designed for 100v commercial installs in malls, arenas, schools, bus stations, etc... but this one has it's transformers removed and happily operates with regular 4-16ohm speakers.
Something I found interesting is that while the pre-amplifer is biased in class A (there is quite a bit of heat comping from those transistors! :D ), the power amplifier is biased in class BC, something I have never hear of until now.
I bought it use as an amp for PA speakers, but I can't, for one simple reason:
It sounds WAY too good. Seriously. I have it playing background music with my B&W DM310s hooked up and laptop as source, and it sounds just brilliant. And that is with the original 30 year old electrolytics! This thing just went way beyond my expectations, has to be hear in order to be believed :D Very good bass, not boomy but very tight and punchy and somewhat "fat" sounding, if that makes sense, very clear and detailed mids and crisp highs. Overall just very musical and detailed. So far I have tried jazz, rock, pop, techno and dubstep with this and it's perfect for every genre! My B&Ws have never sounded this amazing!
I'm going to be taking it apart, cleaning everything, repairing cracked solder joints, maybe a few more things. And it's definitely going to have a phono pre installed! If anyone is interested, I could post some pictures here as I make progress :D
 
Sounds like you lucked into a piece of Soviet-era "Mil-Spec" equipment. From what I've been able to read from threads here, Combloc military equipment factories typically ran off limited quantities of things like amps, tape recorders, turntables, etc, for the "Ruling" class, & a few other lucky individuals. They were built to the same specifications that military equipment was, typically far & above what most "Consumer" grade items were. Its too bad that little of it ever found its way to the West, for from what I've seen, it would easily equal-or BETTER-all but the very highest quality stuff we had here.
 
Sounds like you lucked into a piece of Soviet-era "Mil-Spec" equipment. From what I've been able to read from threads here, Combloc military equipment factories typically ran off limited quantities of things like amps, tape recorders, turntables, etc, for the "Ruling" class, & a few other lucky individuals. They were built to the same specifications that military equipment was, typically far & above what most "Consumer" grade items were. Its too bad that little of it ever found its way to the West, for from what I've seen, it would easily equal-or BETTER-all but the very highest quality stuff we had here.
As a citizen of an ex-communist country, I can confirm that your understanding of Mil-Spec is correct :)
Man, the more I listen to this amp the more I start to like it, best amplifier I've ever heard, and I have heard some pretty high-end stuff :music:
And I love the way it's constructed. If you have it in a rack, parts of it can be removed without even removing it from the rack! Amplifier modules just slide out the front, so does the power transformer assembly. PTF is shielded from the rest of the amplifier.
Pictures are coming tomorrow! :yes:
 
Some of the big boy solid state amps are built that way, class A on the driver board, output stage in class B. It makes for more efficient use of power, and actually doesn't sound bad if you pull it off properly.


I take it the transformers you mentioned converted the standard 8 ohm direct coupled output up to the 100v for distribution lines ?
 
Some of the big boy solid state amps are built that way, class A on the driver board, output stage in class B. It makes for more efficient use of power, and actually doesn't sound bad if you pull it off properly.


I take it the transformers you mentioned converted the standard 8 ohm direct coupled output up to the 100v for distribution lines ?
Well looks like they pulled this one off perfect because this thing sounds incredible :D
100V transformers worked like that, yes. And there is a small transformer on each speaker to get back to regular signal. Some amps like that, those that had 30V lines, you could hook 8ohm speakers right to the 30v line :D I've done it, but it sounds better without the transformers.

There are pictures and schematic on this page: http://www.kn34pc.com/sch/sch_ampl/resprom_emc_1511.html
 
I've worked with the high voltage distribution thing before, I just have never fooled with solid state amps that had that output. Tube amps just have another tap on the output transformer. I guess it makes sense that a solid state amp would have to add a transformer to go from 8 ohms to the 100v circuit.


Looks fairly well made. I like how the circuit sections pull out for repair. Makes fixing things a little easier if you can narrow down the trouble to one area, and then just pull that module out for service.
 
As a citizen of an ex-communist country, I can confirm that your understanding of Mil-Spec is correct :)
Man, the more I listen to this amp the more I start to like it, best amplifier I've ever heard, and I have heard some pretty high-end stuff :music:
And I love the way it's constructed. If you have it in a rack, parts of it can be removed without even removing it from the rack! Amplifier modules just slide out the front, so does the power transformer assembly. PTF is shielded from the rest of the amplifier.
Pictures are coming tomorrow! :yes:

Here in the West, we ran the gamut from absolute JUNK that was barely fit to be brought home all the way to REALLY high-end stuff that rivalled "Mil-Spec" quality. Its all in how much you wanted to/could afford to pay. I started out with, & STILL run today, Yamaha components as my principal "Noisemakers". I had a Philips 312 TT back in '77, which while it was fairly "Cheap & Cheerful" even then, was, & still is, a GOOD TT today.
 
Actually our stuff was similar then, because we also sold everything from BPC to Mil-Spec :D Most Soviet stuff I have had has luckily been Mil-Spec (model numbers that begin with 0) and regular home Hi-Fi (model numbers start with 1) and it's been nice, but I've also had some 3 and 4 class devices that, while still well built), sounded like complete shit!

Well I'm gonna start working on this today or beginning of next week, and post photos here. Initial plan is:
-Clean everything and tidy up soldering
-Recap
-Add shielded input cables from back panel
-Use thick copper cables from amplifier modules to back panels
-Clean existing connectors
-Add multiple RCA inputs on back panel
-Add input selector on front panel
-Add a volume control on front panel
-Add RF filter at AC input (uses regular 3-pin power connector)
-Make a top cover for it
-Add feet on the bottom (got some left over from a Yamaha receiver)
-Add a RIAA amp, when I have more time

That's it for now :)
 
So, some progress :D I've recapped both amplifier modules and changed output wires from ridiculously thin wireds to decent thickness (salvaged power cord :D ) copper wire.
Sound is now much more detailed in HF range, and more punchy. Very nice upgrade :) There also used to be a weird buzz from speakers for about 5 seconds after turning the amp on, but that's gone now.
Changing and rerouting output cables got rid of 90% of output hum.
Next up is adding multiple RCA inputs, shielded copper input cable, a rotary stepped input selector, volume control and headphone output. And this time there will be pictures :D

Whilst using this amplifier, two questions actually came to mind.
First, how do I go about adding a headphone output? Should it tap into preamplifier out into power amplifier, or should it be connected through resistors to power amplifier outputs?
Second, this amplifier was, on the Bulgarian website, described as a class BC amplifier (that is the power/output stage, preamplifier is in class A). It uses two different supply voltages, IIRC 27V and 42V, something in that range. How this works is that from 0-50W output it uses the lower supply voltage and runs in class B, and from 50-115W it uses the higher supply voltage and operates in class C. Would that make it a class H?
Looks like the Bulgarians pulled both class B and C off perfectly, because this thing sounds absolutely amazing in both modes. Using B&W DM310's in my room with it, and I have never heard the B&Ws sing like this, and there is absolutely zero crossover distortion! And when using it with PA speakers, in my back yard, blasting out the whole neighborhood, it still sounds perfectly fine :D

TL;DR Is it class H?
 
Alright, so I've continued working on this amplifier now, and have run into a big problem. Basically what happened was that I took it apart to get rid of all the junk wires and connectors, and now I put it back together I got about 13-15V of DC on each channel.
when power supply ground is not connected to chassis ground, there is no DC offset, but also no sound. Very quiet and distorted sound. Measuring from chassis ground to power supply ground, 13V of DC. After connecting power and chassis ground, DC on speaker outputs. Both relays click on when powered up.
This is the full schematic: http://www.kn34pc.com/sch/sch_ampl/resprom_emc_1511/sch_ampl_resprom_emc_1511.gif
From this schematic, only amplifier and power modules are used. Control/bridging block is not used, and has been removed a while ago.
Amplifier was working today, but now I ****ed it.
What do I do now? :(

Here is something to give a better idea:
sch_ampl_resprom_emc_1511.jpg
Red marks all wiring that has been removed.
Green outline the control/mode block that I removed a while back. NB! This block is only used when this amplifier is paired with the preamplifier unit, and this amplifier will continue normal operation with the control card removed!
Blue shows where the outputs were connected after the power transformers had been removed. Both speaker output grounds are grounded through a 0.1ohm resistor for some reason.
 
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I made a discovery. Offset is there because I'm measuring from speaker + to ground, the DC voltage is on ground! Almost as is the chassis was live!
So it looks like what I have here is a grounding issue between chassis and power. How do I connect them together? How abou floating the power ground with a 1uf film cap?
 
Anyone? I'm actually in kind of a hurry to get this thing up and running, as I might need to use it as a DJ amp at an event.
Both amplifier modules are working fine, the fault is somewhere outside the amplifier modules.
 
makes me wonder if you don't have a short between something in the driver board and chassis. I see a +/- 15v line feeding what looks to be the pre-drivers right at the very front of the audio chain. Also possible that your ground connection in the power supply lifted. It appears that the chassis ground goes to the mid-point of the capacitors in the +/- rail supplies. There is also a center tap on the transformer secondary that ties to ground. Make sure all that is how it should be, and check the actual ground connection between those circuits and chassis. I'm guessing it will be a wire and a screw but I don't really know for sure.
 
I'll double check everything tomorrow, and see if I find any problems.
When I get this issue fixed, this amplifier might actually get re-cased into a Yamaha receiver chassis with custom aluminium face and back.

EDIT: Aww yess, she sings once again! And she has a beautiful voice! :)
It was a grounding issue! I disconnected EVERY ground and made a star ground style connection to the power supply ground, which I then connected to the case. DC offset gone, and after removing all the useless stuff, she sounds better than before! :)
 
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AND ANOTHER ISSUE!
I tested it before with my big PA speakers and maybe I didn't notice it then, but there is a weird buzzing/hum sound on the output! I've changed the input cables, checked all the grounds, checked all connectors, changed the AC input connector (old one had loose pins), and it's still there!
When inputs are connected and music is playing, you can still hear it! And when you switch off power, you can hear the signal goes clean, but not for long because the speaker relays click out about 1 second after power off.
wat do?
 
maybe power feed routing issues? If the mains supply wires are too close to the inputs or the audio circuits it can do this. Twist the wires together if they are not currently twisted. It can help cancel out magnetic fields that produce hum problems.
 
Figured it out, gosh darn stupid ground problems!
I checked my wiring and nothing was running past power lines. Then I checked all my soldering, don't want to brag but it was perfect as always :D
Then I disconnected the audio input grounds from my star ground, and soldered them to the ground pins on amplifier module sockets. Booyah, buzzing gone!
This amplifier has such retarded grounding, connect one ground to the wrong place and you will have problems :(
All is fine now, all is good :)
I've built a three position input selector (two auxiliary and phono, the attenuator is adjustable and I can add/remove inputs anytime :) ) and a volume control.
Also tomorrow I will get my phono board ordered, to build the phono stage. I might actually go for an external phono build, if I can find an enclosure for it. If no, internal it is.
I'll post some pics as soon as I can, the USB port on my SIII is a bit screwed up and I can't transfer files to laptop :(
 
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