Leak Delta 70 - Attempt DIY, Overhaul or buy another amp?

[Sherloq]

Hi All,

I've recently acquired a Leak Delta 70 Amp - this came as part of a nice set up of Sandwich 600s and a Lenco 75 TT. The amp is not working, I also seem to have fused it by using the wrong power setting (I could smell the burning).

Now I'm not sure what to do with it. Should I attempt to fix this? I have little to no electrical knowldge. Should I pay to get it fixed by a leak specialist? He has quoted between £200 to £350 for fix and overhaul. Or should I spend that money on a new (second hand) amp?

Any opinions much appreciated.

Thanks

[jez]

you should be able to buy a working Delta 70 for no more than £50 on e bay.
It could probably be repaired if the mains transformer has not gone (i offer such a service) but it would be cheaper just to buy a working one.

[ashok]

My suggestions.

1. Try to fix it as it is and see how it sounds.

2. Make some mods with modern transistors and see how it sounds.
You might need to make resistor/capacitor ( for stability) changes.

The circuit is available on the Net. My memory of the Leak delta 70 (from the 70's ) was that it was very good......much better than average.

I tried some additional major changes ( which means you can't use the existing pcb's) and it sounds great . What is ' great' is debatable !
Cheers.

[lineup]

Here is the schematic of Stereo 70
Quote:
"the later "Delta 70" Rank-Leak is electrically very similar"
As can be seen, this amp uses qasi complementary output with 2N3055


Website: Leak Circuits

[lineup]

I setup Leak Stereo 70 for sim.
Think I got most of it right.

I changed transistors, used
- BC560C for input
- BD139, BD140 (TO126) to replace those TO39
- Used 2N3055A and then later TIP3055 for output
TIP3055 did lower distortion quite a bit, compared to 2N3055A

First there was something wrong. Did not work.
I had forgot to put in the 470p compensation cap at the input BC560C.
After this everything went well.

I must say I was really surprised to see how good amplifier it is.
Considering so few transistors it is really great!

THD
0.003 % at 1 Watt 8 Ohm
0.012 % at 10 Watt
0.046 % at 60 Watt
Amplifier is called LEAK Stereo 70, but I could not get 70 Watt with reasonable distortion.
But 60 Watt with low dist is not bad.

The blue components in schema are for adjust.
If you do not get the recommended 37 Volt at output node,
then I suggest you do like me. Change the value of R13 in upper left.
The idle DC Current in output is set 30 mA, as recommended.

What I really would change, is the GAIN.
As you can see it takes only ~0.500 Vrms to get close to max output.
So by changing R18 at the bottom, from 2.2k to 1k (or 1.2k)
we get a normal modern Gain level.
(It is not unusual old amplifiers have much more gain, as the level from the sources were different, lower at that time)

[ashok]

What does R11 do ?
I simmed the circuit you have shown using Beigebag and I get the second harmonic as 0.043% at about 1 watt into 8 ohms. That's close to what I measured on a similar circuit I tried out.
The sound is good in any case. But I can see harmonics ( measured ) all the way out to 20 Khz ( about -80dbV or so) on a 1 Khz test signal !

[lineup]

What THD we get, of course, depends on what transistor models we use.
But such big difference is not expected.
Beigebag is a very good sim, what I know. I use MultiSim.
TIP3055 I have downloaded from somewhere. 2N3055A was in MultiSim library.
BD139 BD140 are from Philips website NXP Semiconductors
BC560C I have downloaded.

If your sim is close to reality, you can be happy.

R11 mystery.
I can not tell. We could measure currents.
Because it does balance currents in some way.
R11 puts 2mA extra across R3.
This means we need 2 mA less in U2 BD140.

One thing we could do is to run with and without R11, and observe any difference.

[ashok]

Yeah , you are right about the variation in models causing different results.
Beigbag seemed to have these on their list but the 2N3055 would not appear in the npn power transistor list. I could see it only if ALL npn transistors were selected. Strange. Maybe it isn't a good model after all.

I missed clarifying the fact that the circuit I tried was NOT the exact circuit as you have tried and that could possibly be another cause of ( measured) differences. However it's quite interesting to check it out.

I'll try the R11 sim and see what it does. Might indicate why they put it there.
Cheers.

[jez]

FYI this amp is rated 35W per channel. The 70 bit refers to the output of both channels. They suffer from high order crossover products due to the asymmetrical quasi complementary output stage without Baxandall diodes.
I have owned around six of these over the years and do like them as a period piece and bit of industrial design but I can't say that they sound good..... good for a solid state amp in the late sixties maybe but rather veiled and mushy by modern standards.... and gets harsh as the volume is pushed up.
The Delta 70 is basically the same design but was the version made by The Rank Organisation after they had bought out Leak (they were then made in Idle, Bradford as part of the Wharfedale operation AFAIK). The Delta version is re-styled and rather more robust due to larger heatsinks and a few other detail changes.

[owenhamburg]

Dear Sherloq,

I would get a new one Leak 70 or some thing a little better and renovate the working one with new capacitors, for a slightly more modern and simpler project I would and have renovated Quad 303's the price is higher at about 100 Pounds on Uk ebay and less for a broken one. Get an old sugden a21 if you can. Also when it comes to renovating an amp its nice to have a working one to compare voltages on the schematic.

Good luck what ever you plan to do.

If you do renovate any old amplifier that is working replace all the electrolytic capacitors. Replacing nearly all the capacitors will likely cost around 60 -90 Euro's for a 1970's amplifier. If its not working find new equivalents for all the transistors (buy some extra transistors especially for the output stage (as you may blow up the transistors while your doing the job. 95% of the time what is broken is the output transistors irrespective of make for 1970's amplifiers.