Hi folks
I obtained an Australian Monitor AMC+1202P power amp which is a commercial unit and would like to repurpose it to use as a hifi power amp.
It's a dual mono design, fully discrete with no ICs or opamps anywhere and appears to use good quality parts throughout, so I have hopes it will be a suitable piece to put some effort into.
Here's the schematic:
There are very few caps in it, so I don't think there's much need to upgrade any of them, other than the four 2200uF 63V Jamicon smoothing caps in the power supply stages that appear to have leaked electrolyte and have been discussed here:
Replace leaking caps in Amp PSU stage
I'm wondering if it would be worthwhile to increase the value of the smoothing caps, 2200uF seems a bit small. What would be the benefits of using a greater value to replace them - reduced ripple and increased bass?
The power supply is a bit different to what I'm used to seeing, there is no regulator IC:
The first mod I intend to make after replacing the smoothing caps is to replace the screw terminals for the speaker outputs to proper binding posts as you would find on a domestic hifi amp. I will just disconnect and isolate the 100v and 70v lines - I'll put some heatshrink on the ends of the wires to insulate them and tuck them neatly away. Anyone see any issues with doing this?
I obtained an Australian Monitor AMC+1202P power amp which is a commercial unit and would like to repurpose it to use as a hifi power amp.
It's a dual mono design, fully discrete with no ICs or opamps anywhere and appears to use good quality parts throughout, so I have hopes it will be a suitable piece to put some effort into.
Here's the schematic:
There are very few caps in it, so I don't think there's much need to upgrade any of them, other than the four 2200uF 63V Jamicon smoothing caps in the power supply stages that appear to have leaked electrolyte and have been discussed here:
Replace leaking caps in Amp PSU stage
I'm wondering if it would be worthwhile to increase the value of the smoothing caps, 2200uF seems a bit small. What would be the benefits of using a greater value to replace them - reduced ripple and increased bass?
The power supply is a bit different to what I'm used to seeing, there is no regulator IC:
The first mod I intend to make after replacing the smoothing caps is to replace the screw terminals for the speaker outputs to proper binding posts as you would find on a domestic hifi amp. I will just disconnect and isolate the 100v and 70v lines - I'll put some heatshrink on the ends of the wires to insulate them and tuck them neatly away. Anyone see any issues with doing this?
Attachments
Here's some pictures of the innards, any suggestions for mods/upgrades are much appreciated.
It's biased as a class B device, but for my purposes, I won't be using more than maybe 15-20w max per channel rather than the 60wpc it is capable of, so I'm wondering, could the bias be adjusted so it runs class A for the first few watts in order to make it sound a bit better? The heatsinks are quite substantial and it has thermal protection circuits, so I doubt it would melt.
It's biased as a class B device, but for my purposes, I won't be using more than maybe 15-20w max per channel rather than the 60wpc it is capable of, so I'm wondering, could the bias be adjusted so it runs class A for the first few watts in order to make it sound a bit better? The heatsinks are quite substantial and it has thermal protection circuits, so I doubt it would melt.
Attachments
While its built to a high industrial spec. this LINE ( factory broadcasting ) amplifier does not lend itself naturally to "hi-fi " .
It wasn't designed for high quality audio.
It does have quality expensive parts including the high quality metal film resistors /heatsinks etc .
Its extremely well built to industrial standards and must have cost a lot new but ---personally-IMHO --I would use the case and parts and build an audio power amplifier of low distortion.
I know that's not what you want to hear but if anybody thinks that you can use the same circuit for real hi-fi ---well I am willing to be impressed as long as they give a detailed explanation ?
It wasn't designed for high quality audio.
It does have quality expensive parts including the high quality metal film resistors /heatsinks etc .
Its extremely well built to industrial standards and must have cost a lot new but ---personally-IMHO --I would use the case and parts and build an audio power amplifier of low distortion.
I know that's not what you want to hear but if anybody thinks that you can use the same circuit for real hi-fi ---well I am willing to be impressed as long as they give a detailed explanation ?
Hi Duncan
That's the kind of feedback I need, cheers.
Removing the amp modules in order to reuse the transformers, heatsinks, case etc is a distinct possibility as I already have some class A amp modules I constructed but never implemented because I didn't have a suitable chassis and heatsinks.
That's the kind of feedback I need, cheers.
Removing the amp modules in order to reuse the transformers, heatsinks, case etc is a distinct possibility as I already have some class A amp modules I constructed but never implemented because I didn't have a suitable chassis and heatsinks.
Yes, the specs tell you it's no Hi-fi amplifier and the brochure details don't claim it to be a stereo model, despite the similarities.
0.5% THD, bandwidth limited by the PA grade driver and output transformers and only -82dB A weighted noise figure don't quite excite so you are down to just the case, heatsinks, connectors and power transformer - the essential and expensive bits really, so all is not lost.
0.5% THD, bandwidth limited by the PA grade driver and output transformers and only -82dB A weighted noise figure don't quite excite so you are down to just the case, heatsinks, connectors and power transformer - the essential and expensive bits really, so all is not lost.
So there's no point making any modifications to the existing circuitry and best to just rip it out?
I can always salvage the TIP35C output trannies, they might come in handy at some point.
I can always salvage the TIP35C output trannies, they might come in handy at some point.
The frequency response of the proposed amplifier is linear from 20 to 50,000 Hz. The scheme was developed back in the 50s of the last century.
An externally hosted image should be here but it was not working when we last tested it.
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Ooh, that sounds interesting, I shall have to send Susan a PM and ask for her opinions on the suitability of those output transformers. I might even have some power MOSFETs that would be suitable, although I think you need to use matched pairs.
First, check the different design and write your specification then get a price and availability offer for Susan Parker's or similar transformers. Then speak to your bank manager about a loan to pay for them.
I don't see why building a Zeus type amp would be all that expensive given that I already have some output transformers that could be suitable and Susan has demonstrated that you can build amps of that type using mains toroid transformers instead of custom wide bandwidth audio transformers. I have a pile of mains toroid trafos and among them may be some suitable to pair with the output trafos from this Australian Monitor amp.
Zeus Amp - Toroid Transformer Version
I haven't read all 174 pages of the monster thread on Susan's amp design and don't really understand the technical details, so I'll have to ask Susan for some guidance before I know if I have suitable trafos and if I want to build a Zeus amp.
Zeus Amp - Toroid Transformer Version
I haven't read all 174 pages of the monster thread on Susan's amp design and don't really understand the technical details, so I'll have to ask Susan for some guidance before I know if I have suitable trafos and if I want to build a Zeus amp.
If you read her threads, Susan's transformers were mostly specialties - custom designed and wound. As an engineer, she will know how to specify them and I imagine, save quite a bit of money on the design and sourcing costs. Your amplifier is quite different in concept to hers and being a PA amplifier, the audio transformers are not just like any 50Hz power transformer. For linearity and low noise beyond 20kHz, you can't use just any grade of iron alloy or construction technique either.
As I understand the situation, you'd be spending a lot to improve on the amplifier you have, just to render it compatible with 8 ohm loads at full power. As advised earlier, it's output impedance is high - determined by the 100V PA line output transformer which is intended to couple to multiple loudspeakers at high impedance via a daisy chain of long, light gauge cable to remote PA speakers spread throughout a venue, building etc. These speakers usually have small, individually rated and adjusted audio matching transformers in each box to ensure an appropriate balance of volume levels at each each location. Here's a full explanation of PA systems:
Understanding 100V Line Distributed Speaker Systems. - Redback Audio
Essentially, if you expect hi-fi from this type amplifier, you'll probably have to replace the output transformer with one that is custom designed for the power and actual speaker impedance(s) you plan to use.
As I understand the situation, you'd be spending a lot to improve on the amplifier you have, just to render it compatible with 8 ohm loads at full power. As advised earlier, it's output impedance is high - determined by the 100V PA line output transformer which is intended to couple to multiple loudspeakers at high impedance via a daisy chain of long, light gauge cable to remote PA speakers spread throughout a venue, building etc. These speakers usually have small, individually rated and adjusted audio matching transformers in each box to ensure an appropriate balance of volume levels at each each location. Here's a full explanation of PA systems:
Understanding 100V Line Distributed Speaker Systems. - Redback Audio
Essentially, if you expect hi-fi from this type amplifier, you'll probably have to replace the output transformer with one that is custom designed for the power and actual speaker impedance(s) you plan to use.
Sorry Ian, but you're not correct about the output of this amp. Take another look at the specs and photos - it has both line outputs (100v and 70v) and low impedance 4ohm outputs, which are for connecting normal speakers.
Have you taken more than a cursory look at Susan's work? She has used a variety of different transformers, not just expensive ones. You can see some of them bere:
Zero Feedback Transformer Audio Power Amplifier
Have you taken more than a cursory look at Susan's work? She has used a variety of different transformers, not just expensive ones. You can see some of them bere:
Zero Feedback Transformer Audio Power Amplifier
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It does, like most high power PA systems have but not many 4-8R monitor windings are capable of the the full power of the amplifier, as I found many years ago when I blew the 4R windings on a customer's amplifier and it cost me a lot more than the job was worth. So much for trying to be clever.....
Are you accusing me of trying to be clever?
If so, then I would kindly ask you to not bother polluting my thread further.
If so, then I would kindly ask you to not bother polluting my thread further.
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This is a specialized amplifier with a transformer at the input and output of a powerful stage. Monaural (1 channel joins 2). HiFi quality does not provide - tweaks will not help. Only complete rework using parts, power supply, heatsink and box. New PCB.
Cheers, I am considering options for what I can build using the chassis, heatsinks and possibly the transformers.