what would you do with this 1000w beast? calling power amp experts
I bought this over the Christmas period. I could not find any info, so bought blind! It was not a lot of money. I liked the size of the transformer! and that is was passively cooled. It uses 3x2 Motorola MJ15003/4 and a pair of MJ15001/2 per channel. It is rated at 250w into 8 x 4 channels. Made by a Swedish company called Amtech. The only thing I could find was they had made what seemed to be a high quality mixing desk, so I sort of hoped this may be a high quality amp for studio use.
I have since tracked down a schematic and circuit diagram, attached.
My intended use is to drive via a dsp active crossover (dbx venu360) a pair of 8" bass units on my hybrid Innersound Isis ELS, and a pair of Linkwitz open baffle subs, in a domestic hifi environment.
Any thoughts if this is with bothering with, I am worried by its PA spec, maybe someone could suggest swapping out parts or boards to make a better unit? ideas?
I bought this over the Christmas period. I could not find any info, so bought blind! It was not a lot of money. I liked the size of the transformer! and that is was passively cooled. It uses 3x2 Motorola MJ15003/4 and a pair of MJ15001/2 per channel. It is rated at 250w into 8 x 4 channels. Made by a Swedish company called Amtech. The only thing I could find was they had made what seemed to be a high quality mixing desk, so I sort of hoped this may be a high quality amp for studio use.
I have since tracked down a schematic and circuit diagram, attached.
My intended use is to drive via a dsp active crossover (dbx venu360) a pair of 8" bass units on my hybrid Innersound Isis ELS, and a pair of Linkwitz open baffle subs, in a domestic hifi environment.
Any thoughts if this is with bothering with, I am worried by its PA spec, maybe someone could suggest swapping out parts or boards to make a better unit? ideas?
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btw to a fairly untrained eye, it all looks original and i can't see any signs of damage, repair work etc. It was filthy dirty, so spent a bit of time giving a bit of a clean...did not go as far as a full disassembly and wash though.
If it is working, the first thing to play around with is the input OP-AMP. There are MANY newer and better choices here. - Still, the TL081 is not all that crappy: https://www.ti.com/lit/ds/symlink/tl081.pdf
Hook it up - and if it works, enjoy... It is a beast indeed - but may not be worth fooling with too much.
D.
Hook it up - and if it works, enjoy... It is a beast indeed - but may not be worth fooling with too much.
D.
I am a bit worried by the frequency response, quoted at 30-15000. would you recommend a simple drop in opamp, would just a simple swap to TL071 be worth while, only a few cents?
there is a quite a bit of mechanical hum when I power it up. there is rubber between the toroidal and the clamp. Maybe the rectifiers?
Its certainly a nice looking piece of professional engineering , I even recognize the black heatsink --same era as the ones I used to build JLH,s 80 watts/channel mosfet amplifier ( 1984 version ) .
The TL081 at the input wasn't the top audio IC of the time in that range so while it might be used for "studio use " back then it certainly wasn't in the same league as any JLH design nor D.Self of the time ( 1980,s ) .
If it was mine I would redesign the whole circuit as its a nice piece of engineering for which you would pay $$$$ of dollars for nowadays.
The TL081 at the input wasn't the top audio IC of the time in that range so while it might be used for "studio use " back then it certainly wasn't in the same league as any JLH design nor D.Self of the time ( 1980,s ) .
If it was mine I would redesign the whole circuit as its a nice piece of engineering for which you would pay $$$$ of dollars for nowadays.
That piece of rubber was there for two reasons-
#1- to stop shorted turns in the transformer .
#2- to reduce mechanical hum .
Definitely replace it with a thicker -modern synthetic plastic type -don't buy cheap !
#1- to stop shorted turns in the transformer .
#2- to reduce mechanical hum .
Definitely replace it with a thicker -modern synthetic plastic type -don't buy cheap !
I was planning on replacing the sockets. change the input to female - strange they are male? and change the outputs to speakon. such simple changes are in my reach. What modern replacement would you recommend for the huge transformer?
I would keep the transformer its worth a good few bucks and its a toroid .
Change the main capacitors and I notice the small maroon capacitors they are the same type as used in the Quad 33 pre-amp.
I personally don't like those colored striped polyester capacitors.
Modern RCA sockets are available --don't buy those cheap tinned ones ,the tinning wears off after more than 10 years - buy quality THICK gold plated ones in heavy duty /industrial types.
Funny you asked about me redesigning it for you , after 7 years away a poster on Head-Fi PM,d me asking me to upgrade his Stax energizer which I had modified , had to turn him down as I don't have the time now .
Probably has nothing to do with the Swedish company but a British version with the same name exists on UK Company records but they don't make amplifiers .
Two officers exist - the director is German and lives in Germany - Dieter Rothland.
Change the main capacitors and I notice the small maroon capacitors they are the same type as used in the Quad 33 pre-amp.
I personally don't like those colored striped polyester capacitors.
Modern RCA sockets are available --don't buy those cheap tinned ones ,the tinning wears off after more than 10 years - buy quality THICK gold plated ones in heavy duty /industrial types.
Funny you asked about me redesigning it for you , after 7 years away a poster on Head-Fi PM,d me asking me to upgrade his Stax energizer which I had modified , had to turn him down as I don't have the time now .
Probably has nothing to do with the Swedish company but a British version with the same name exists on UK Company records but they don't make amplifiers .
Two officers exist - the director is German and lives in Germany - Dieter Rothland.
David,
That is a superbly well-built beast indeed! I suggest you modify it as little as possible to retain its professional construction.
Yes, do change the inputs to female XLRs, and yes, do change the outputs to speakON. Also, do get rid of (i.e. simply solder through) the split/bridge switch (unless you will be using it), but NO, don't replace that lovely transformer, unless there is something seriously wrong with it. A little mechanical noise? Yes, there would be, with a 1.76 kW toroid happily humming away inside. @;-)
Seriously, if you're thinking of replacing the power supply with a switched ditto just to get rid of some hum, sell the beast on and start afresh.
The specs say the S/N ratio should be 100 dB, so if there is any audible hum on the outputs (with the input shorted; do use a cheap little test speaker), you will probably need to replace the power supply capacitors. A little extra capacity won't do any harm, but make sure they are mechanically compatible to save yourself a lot of aggro.
I suspect the 30 Hz to 15 kHz frequency response spec is a ±n dB power bandwidth value only, but it could also be a hard limit to avoid PA problems. To be sure you would need to measure it (and then look closely at the circuit diagram to see what changes might be needed). The 0.7% HD value at clipping point doesn't look bad, either.
I don't think changing the OpAmp will affect the audio bandwidth/quality to any noticeable degree, so best leave it alone is what I say.
Have you had a proper listen yet?
Marcus
sorry, read back, I have no intension to replace the toroid, I was referring to the rubber above and below to reduce the hum.
I haven't connected up, due to the odd sockets, my XLRs are the std ones! so I have new parts arriving soon to make up the correct cables. Then I will try into some old speakers first before trying it with my innersounds.
I haven't connected up, due to the odd sockets, my XLRs are the std ones! so I have new parts arriving soon to make up the correct cables. Then I will try into some old speakers first before trying it with my innersounds.
those ROE capacitors should be replaced, they turned up in a bunch of revox amps that i serviced and upgraded, and most were out of spec or causing problems. what are the IC's?- they would probably benefit from replacement
These ROEs dry out and completely loose capacitance. I learned this with my Revox A77 long time ago.
So you spent $100 on an excellent and perfectly functioning amp and want to spend $2000 to "improve" it.
I don´t see the logic.
I see the logic OK. It's what you read parroted on the internet as sage advice about any outdated amplifier, whether it has age or fault related issues or it's just a quest for knowledge. It's also the shotgun approach to repairs in disguise and often unfortunately, ends in shotgun-like damage and tears.
Unless you're an experienced restorer who knows the risks in what they're doing, only fix what is necessary to return the amplifier to original spec. and don't modify or upgrade semis or components until you have listened carefully and directly compared it in restored form with other products that you like the sound of.
Rolling opamps that have principally a DC or low gain signal function won't make any useful difference to sound. You can't easily or honestly compare them in the same amplifier so don't waste time and money on gilding lilies, whatever the sage opinions.
Liking an amplifier or other device even though its actually worse for distortion is another matter. It would be remiss to ignore a large number of our community who actually prefer lots of "musical" harmonics in their audio but are probably loath to admit it to their peers or associates who follow only textbook precepts.
Unless you're an experienced restorer who knows the risks in what they're doing, only fix what is necessary to return the amplifier to original spec. and don't modify or upgrade semis or components until you have listened carefully and directly compared it in restored form with other products that you like the sound of.
Rolling opamps that have principally a DC or low gain signal function won't make any useful difference to sound. You can't easily or honestly compare them in the same amplifier so don't waste time and money on gilding lilies, whatever the sage opinions.
Liking an amplifier or other device even though its actually worse for distortion is another matter. It would be remiss to ignore a large number of our community who actually prefer lots of "musical" harmonics in their audio but are probably loath to admit it to their peers or associates who follow only textbook precepts.
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If you want to try different op amps put a socket.
But you may have to change the gain and input resistors.
In my opinion, 5532 and TL 072 are good, the 072 had a higher slew rate than most chip amps. That tells you how fast the chip responds to changes in input.
S/N ratio and all show up when the original recording is digital, the older record or cassettes had only 60dB from lowest to highest
The newer op amps have pretty much the same or lower slew rate, and most are pin for pin compatible.
When I was building a phono pre amp, I put a socket, and tried LM358, 4558 and 072.
072 was best, but I had to reduce in and out resistors.
And some of the new chips are in SMD, not available in DIP at all.
But you may have to change the gain and input resistors.
In my opinion, 5532 and TL 072 are good, the 072 had a higher slew rate than most chip amps. That tells you how fast the chip responds to changes in input.
S/N ratio and all show up when the original recording is digital, the older record or cassettes had only 60dB from lowest to highest
The newer op amps have pretty much the same or lower slew rate, and most are pin for pin compatible.
When I was building a phono pre amp, I put a socket, and tried LM358, 4558 and 072.
072 was best, but I had to reduce in and out resistors.
And some of the new chips are in SMD, not available in DIP at all.