Leak Delta 30 getting hot.

[PaulR]

I bought this amp a few years ago and had to do quite a lot of work to get it working.

I haven't used it much until now when I thought it could take a turn. I firstly used it with a pair of those small Mission made for Denon speakers and all was well. I thought I would try some B&W DM4s and found that it is getting hot. I then tried a couple of old Wharfedales and it ran cool.

Although it seemed unlikely I wondered whether the crossover caps in the B&Ws were leaky so I have recapped one of them. The amp gets hot running the recapped B&W with one of the Denons. The volume is the same between the two types of speakers.

Has anyone had the same experience with one type of speaker making an amp run hot, and can anyone explain this? I could understand it if one speaker required a lot more power but the volume output is the same with the different types

[Nickthedentist]

If the DM4s were less sensitive than the Mission/Denons, that might account for it, but I'd be surprised if that were the case.

I wonder whether it's going unstable and oscillating?

[Sideband]

I presume it's nothing to do with impedance.....4 ohm and 8 ohm? I have one of these amps but I can't remember the impedance of the speakers I'm using.

I think Nick might have a good point about oscillation although why one set of speakers cause it and another pair don't

[Radio Wrangler]

It's scopetime!

Every home should have (at least) one oscilloscope.

David

[PaulR]

Instability sounds like a good idea, Nick. I did replace the output transistors as there were some very odd ones there. I will untangle the 'scope and bring it down.

Edit - Both sets of speakers are 8 ohm

[PaulR]

Yes, it is oscillating. It does it with or without an input and with no speakers attached, or with the B&Ws. The Mission ones damp it for some reason.

Next question - how do I stop it?

[dave cox]

Two options,

(1) Get some 'original, manufactured at a similar time, output transistors'.
This is probably the best option, it seems this is using 2N3055 but you need to identify which of it's many incarnations the original design used.
Modern branded 2N3055 will almost certainly cause problems.

(2) If my memory serves my right, the power transistors are on the end of lengths of wire as they are remotely mounted.
You could try introducing a 10R carbon 1/4W resistor between the base wire and the base connection, right at the transistor with the LEAST length of wire possible to the transistors base.
This is last resort and don't blame me if it makes it worse so take precautions when you power it up

dc

[PaulR]

Thanks Dave

There already was a 10R resistor on the pcb so I increased this to 20R. Unfortunately it hasn't helped.

I think that the possibility of getting original spec 2N3055s is pretty low - unless I cannibalise another amp

[PaulR]

Doing some more research I realised that the point was to get the resistors at the transistor end of the lead. I returned the resistors on the board to 10R and put more 10Rs at the transistor end of the leads. Unfortunately the high frequency oscillation is still there.

Edit - There do seem to be some transistors described as vintage old ones available. I have risked buying 4. Fingers crossed!

[Ted Kendall]

MJ15003s, perhaps? Some claim them to be slow enough to make a Quad 303 stable.

[Michael Maurice]

What about putting some small capacitors say 50 - 100pF across the B-E terminals of the o/p transistors?

[Radio Wrangler]

Originally Posted by PaulR

There already was a 10R resistor on the pcb so I increased this to 20R. Unfortunately it hasn't helped.

These may simply be at the wrong end of the connecting wires. Stoppers need to be very close to the transistors concerned. Newer, faster transistors just see long wires as coupling elements.

David

[PaulR]

Thanks all.

I did later realise that the intention was to have the resistors at the transistor end of the lead, David, so I tried them there but it didn't help.

I will try capacitors as you suggest tomorrow, Michael.

Tim, I read about MJ15003s, but decided to buy some used 2N3055s. I actually have an Armstrong 600 series amp which seems to have similar problems after the original transistors had to be replaced so if the capacitors help with the Leak I could try the vintage transistors on that. I think they are a bit easier to get at on the Armstrong - they are a but buried on the Leak. Not a lot of thought given to heat dissipation on that.

[PaulR]

I haven't got any capacitors with such low value so I have ordered some.

[stuarth]

Since the amp oscillates with no load, and the Mission speakers damp the oscillations, might an RC network between the amplifier output and ground help?. The phrase "Zobel Network" springs to mind but I know this phrase is sometimes used incorrectly. David RW could elaborate (but don't mention Faraday cages, another misused phrase).

Stuart

[dave cox]

I suppose the other option is to make the dominant pole more dominant !
This looks to be at C31 (400pF) - this seemed a large value to me but its not being magnified by much miller effect so it might be worth increasing is a bit, maybe to 560pF but see what other say.

This kind of problem can be a REAL to fix, hence my original suggestion that 'original' devices are SO much easier
Somewhere on this site is a thread with me trying to 'fix' a rather unforgiving Sony amplifier that liked to oscillate rail to rail

dc

[PaulR]

I can certainly try that, Dave. I have ordered some old transistors so can try them if all else fails. They are mounted on a vertical heat sink in the middle of the amp, one above the other so are not easy to get at so I am hoping that it won't prove necessary

[Radio Wrangler]

As I seem to have been invoked...

I suspect that it may be differences in the crossover networks of the different speakers which are either encouraging or damoing HF instability.

What's called a Zobel network (Well, he did so many networks, he was one of the kingpins of network theory and early filter design) is a sort of diplexer. It makes the amplifier see the speaker in the audio range, and above it it presents a nice, dull resistor load to the amp.

The full monty is an inductor en-route to the speaker with a damping resistor across this inductor, and then a series resistor/capacitor shunting the amplifier output to ground.

Values need only be rough. Something like a 2W 10 Ohm resistor with 7 or 8 turns wound over its body, soldered at each end is common, and 10 Ohms series with 0,1uF for the shunt element.

Some cheapskate amps just had the shunt element, but didn't do the full job.

Some really cheapskate amps missed the whole thing, and some like the Leak date from before people knew about this trick... and one very snobby brand eschewed them on religious grounds, acquiring a reputation for smoke with some esoteric speaker cables.

If your Leak hasn't got these components, they're worth adding.

David

[Sideband]

Service information should be available using the download link, top right of this page.

I reckon you should fit some genuine 2N3055's first. There are so many fakes about, there is no guarantee that new ones are the real deal.

[stuarth]

Thank you for the clarification David. I remember a Mullard book from the 60’s where most power amps were shown with the shunt element of the Zobel network. Since the OP’s amp was tamed by connecting the Mission speakers, it must be worth trying such a network, if it works it’s a lot easier than sourcing and fitting slow power transistors.

Stuart