Identification of unknown PA amplifier
 

Hi All
Hoping someone would have any idea who manufactured this PA amp. Mosfet with cloth wrapped toroidal transformer. XLR inputs plus balance line. Ouput think is around 150watt per module.

Not been able to find any information on it date 1970s.
Just added new psu caps all blown and new heatsink grease plus all F5 fuses had blown due to the psu caps having leaked and dead as doornails. But now all checks out okay. I'll be making up some cables today to see how it fairs sound wise.

The nearest I can get it re make is Cambridge Audio!

Cheers Chris

Re: Identification of unknown PA amplifier
 

Couple more pics

Re: Identification of unknown PA amplifier
 

Hello Chris. The boards are OMP as sold in electronics magazines late 70's early 80's. They later were sold by BK Electronics and the casework is very much the same design they used for their mosfet range.
Never seen that style of front panel before though.

Output was 100w into 8 and 115w into 4 ohms although it could be a bit more with a larger power supply.

David.

Re: Identification of unknown PA amplifier
 

Cheers David
Yeah definitely been manufactured by a company or a very good engineer with the tools to build a quality cabinet. Its all aluminium around 5mm thick apart from the surround on the face plate and top and bottom plates.
Plus it has 14 heatsink,s in total. Quite a lot.
Yeah the balance I thought was an unusual feature next to the input. The face plate is perspex and behind are two rectangular cut outs so not sure if meters may have been in there no evidence to suggest there was or cut out if you wanted to add amp/v meters in there as an extra feature.
Looking at some of the Cambridge designs made me think along those lines of manufacture or it was a bespoke kit made up for the DJ to his spec.
Well thats a start thanks David.

Re: Identification of unknown PA amplifier
 

That front panel (if you took the sockets and handles away) looks like a cross between a Crown DC 300 A and an HH S500 D.
It's funny that the HH copied the circuit of the Crown more or less.

The module makers (OMP) did offer a rectangular LED VU meter as an optional extra and these are used on the BK range.

Oddly, I think I saw this amp for sale but talked myself out of it. Hope you can make some good use of it.

David.

Re: Identification of unknown PA amplifier
 

Thanks David got this in a job lot from a guy who used to be a DJ had connections with Bob Ray the DJ. Shame you don't have the advert for it could have done a comparison.

Re: Identification of unknown PA amplifier
 

Interesting some similarities there but not quite the same at all. The other quirk to this one is the toroidal is bolted to a vibration bar which you can undo and slide back and forth to get to the components underneath. Not seen that on any so far. Odd one. Funny enough I also got pair of those in the job lot. Both working spot on bk modules.

Re: Identification of unknown PA amplifier
 

The difference in those 2 you have there is that they are later mosfet designs and the amps in your original post are bipolar.
The 2SD424 output devices are more or less the same as 2N3773 transistors.

The mosfet outputs will have a P channel and N channel output device so not comparable with your other amp in post 1.

These modules have come up on the forum before and there was no exact ID.
I expect they made slightly different versions for OEM's.

David.

Re: Identification of unknown PA amplifier
 

Just struck me when you mentioned the bk range as that one is definitely homemade piece of kit. So would not expect that a manufactured piece of kit. Whereas the other definitely is. I'll have to keep looking round I'm sure one will pop up somewhere.
Thanks for info on differences.

Re: Identification of unknown PA amplifier
 

I was thinking David what pre amp would you suggest for the first amp. Basically for tone controls.

Re: Identification of unknown PA amplifier
 

In the late 70's everybody and their father brought out MOSFET audio amps and assembled boards and kits. These were all started by Hitachi releasing a range of power MOSFETs and some applications notes. They suddenly made reasonable audio amplifiers a lot easier.

It's likely that the power amp stages in your unit have not strayed terribly far from Hitachi's applications note circuits.

David

Re: Identification of unknown PA amplifier
 

There was also a project in one of those little Babani books by R.A. Penfold that used them.

Re: Identification of unknown PA amplifier
 

Oh deary me well voltages looking good all round same readings on both boards. Made up cables hooked up PA speakers no hum not got round to doing leads for cd player yet. Job for tomorrow. Taking reading off 8223 tip 30c looked good cat walks in slipped with probe hit the heat sink whilst still touching leg then great big spark no smoke. ****** stupid or what. Not getting a reading now so hoping I've not shorted it. Can't seem to find replacement one similar surface mount only two legs connected to pcb other leg not connected. I'll double check it again tomorrow. All new to me don't normally touch fet amps/solid state gear I'm into refurbing valve gear. Not used to seeing voltages so low more used to seeing 400v plus and a spaghetti junction of wires and resistors and caps etc all buried. These are lot easier to work on blimey quick look at board pretty much see what's what straight off.
Anyhow hoping not blown this tip 30c.

Re: Identification of unknown PA amplifier
 

My sympathy. I did something similar on a Quad 405 a while ago just when I was pretty satisfied I'd fixed it and was just doing some final checks. Took me a while to rectify my own damage!

In a situation like this when the damage to transistors is unknown, I've found the 'diode test' facility on a DMM very useful. Use it to do a 'power off' check that each transistor has two working junctions, shown by a forward voltage between around 0.6V and 0.8V. If so, the transistor is likely to be OK, with a good base-emitter junction and a good base-collector junction. If in doubt about the connections, experiment until you confirm the two junctions. You won't damage anything with the power off.

Martin

Re: Identification of unknown PA amplifier
 

Cheers Martin I'll do that tomorrow ��.
Cat always does it when I seem to be probing around plus I should have been using the short tip probe instead of the long one. Would have just hitted the insulator around tip.

Re: Identification of unknown PA amplifier
 

One op amp and big butch drivers complimentary outputs looks like an overblown Texan amp to me.
Trev.

Re: Identification of unknown PA amplifier
 

Don't just rely on two apparently OK diode junctions, check that collector and emitter are not shorted as well!

Re: Identification of unknown PA amplifier
 

I think it's getting a bit confusing now.

The amps in your original post are BIPOLAR not Mosfet. As I mentioned before the output devices being 2SD424 as used in the Quad 405.

The driver transistors have the middle leg connected through the tab being bolted to the pcb.

As for pre amps. These amps were designed to be used with a mixer feeding them but any line level 0dB or 0.775v input will do.

I often test power amps using a portable cd or minidisc player from the headphone output. The older Sony players sound great with the bass boost function engaged. Just set the volume level around half way and use the amp level knobs to adjust the volume to the speakers.

David.

Re: Identification of unknown PA amplifier
 

The bipolar output transistors do not seem to be matched, look closely different type numbers.
Hard to see but there's not enough semiconductors on the board.
Trev

Re: Identification of unknown PA amplifier
 

Well I stand corrected !

Hello Trev by the way.
Yes you're right the other one is a 2SB554.
I should have looked harder.

David.