redesign of leach amp pcb for integrated TO-247 output devices

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#82
No split feedback

Hi,

Well you should be ok, but be sure to watch for any oscillation.

also you might want to make some hi freq compensation in the feedback path, since there is som peaking at about 50 hkz.

I can post simulationresults if you want them. My leach has the onsemi version of the 2sa3281/1302 pair called mjl3281/1302.

I have had no stability problems with it.

good luck

\Jens
 
#83
jarek said:
What do you think about omitting 'split feedback' and picking it all exactly from the output?😕
Click to expand...


In the beginning, Leach didn't use feedback from the driver stage - only from the output. He began using feedback from the driver for a reason - to increase stability. If you aren't driving highly capacitively loads, removing it should be fine. I use the Version 3 amp and have no problems with my equally old Speakerlab 4's. Still, you have to assume the risks.

I think the base resistors for the output transistors can also be safely eliminated, unless your wires are relatively long. While the Leach Amp is designed to be as bulletproor as possible, that reggedness does include some complications that may only be necessary in a fraction of amp's that are built.

You might build the amp per plans, then once all is working and settled down, you can go in and start removing/changing things to see how things work out.

Jeff
 
#84
Jeff R said:

In the beginning, Leach didn't use feedback from the driver stage - only from the output. He began using feedback from the driver for a reason - to increase stability. If you aren't driving highly capacitively loads, removing it should be fine. I use the Version 3 amp and have no problems with my equally old Speakerlab 4's. Still, you have to assume the risks.
Click to expand...

Exactly, here is the original 1976/77 article from Audio magazine:
http://users.ece.gatech.edu/~mleach/papers/lowtim/feb76feb77articles.pdf

The one pictured in the article is still in use today in his office. He plays it 8-10 hours a day, and everyday since he built it.

--
Brian
 
#86
SteveA said:
Anybody-
Can one use the mjl 3281/1302 or the 2sa1943/2sc5200 pairs be used with the 4.5 boards available from Prof Leach? And if so, does the "diode array" need to be implemented (on the heat sinks)?
thanks
SteveA
Click to expand...

The MJL3281A/1302A devices should just be drop in replacements for the older MJ15003/4 devices, with no other modifications needed in the rest of the circuit. I would leave the Vbe multiplier bias circuit as it is with the diodes on the heatsink. You could switch it over to using a Vbe multiplier transistor, but Dr. Leach said that this might cause stability problems.

--
Brian
 
#88
Number of Output Transistors

Hallo Jens,

quote:
--------------------------------------------------------------------------------

I can post simulationresults if you want them. My leach has the onsemi version of the 2sa3281/1302 pair called mjl3281/1302.

--------------------------------------------------------------------------------

As you have simulated/analysed the amplifier could you please advise how many MJL3281/1302 pairs (or perhaps 2SA1216/2SC2292 pairs) may be used in the output stage of the standard (Leach 4.5) version without overloading the drivers or pre-drivers.

Thanks,
Zvon
 
#89
Output transistors

Hello Zvon

I have simulated up 5 pairs of output transistors, and I'm running the amp with 5 pairs right now.....

I use the mje340/350 transistors for pre driver and VAS,
the drivers are mje15032/15033.

I have tested into 4 ohms continously without any problems.

The drivers only get a little warm when using a small heatsink.

I have modified the protection circuit to a max shortcircuit current of 12 amps. The SOA is tested with simulations and the max power in each output transistor is 150W (peak) with rails at 55V.

Hope it's ok for you, if not write again, or I can send you the files.

I'm using p-spice 9.23 at school, since the student version can only simulate up to 15 transistors.

\Jens
 
#90
Output Transistors & Drivers

Hallo Jens,

Thanks for the prompt reply and offer to send the p-spice files. Unfortunately, I'm not an EE :bawling: and don't know how to use the simulation software :bawling:.

Presently I'm trying to build a pair of monoblock amplifiers with good stability and current drive capability to drive my DIY full range electrostatic speakers. These ESL's are 0.6m wide by 1.2m high (each), 1:100 step-up transformer and present a difficult load to drive. Some (majority) of mid-fi commercial receivers or amplifiers are not happy to drive them as the protection circuits kick-in at only moderate sound levels. The better quality amplifiers (I have tried Plinius and ME) drive the ESL's without problems and do sound very nice, however, I cannot afford them.

The Leach amplifier appears a good DIY option as it is (or should be!?) stable into the capacitive (ESL) loads. To reduce the output transistors load I thought that their doubling (4 pairs) or tripling (6 pairs) may be a good idea. The main concern was that this modification would mitigate the problem and overload the drivers.

The current plan is to use six (6) pairs of 2SA1216/2SC2292 (with minHfe=40 compared to MJ15003/4 min Hfe=20) output transistors per monoblock. For the drivers I have MJE15030/31 which may be fitted with large heatsinks. In regard to the pre-driver and Vas transistors I have problems sourcing both NPN and PNP components from a single manufacturer (which would be preferred) and matching them. So I'll switch to the Motorola's MJE340/350 which are easier to heatsink as well. In regard to the power supplies, I'm stuck with a pair of 2x35V 500 VA toroids.

Do you and other resident experts think the above setup would work OK :xeye: ? Should I increase the drivers and pre-drivers bias ?

Your comments and suggestions will be greatly appreciated and incorporated 🙂 .

Thanks
Zvon
 
#92
Step-up Transformer

John,

My ESL step-up transformers are large core, 1:100 ratio, heavy beasties wound by a local transformer winding company in Fremantle some 5-6 years ago.

I understand that ER Audio now offers an improved verison of these transformers with 1:90 step-up ratio.

You can always use two 1:50 transformers (from Barry Waldron in US) and connect primaries in parallel and secondaries in series and you have 1:100 transfomer (and a nasty amplifier load).

Regards,
Zvon
 
#93
Output Transistors

Is the Leach amp modification proposed in one of the previous posts OK or I'm way of the mark? :dunno:

(The mod is substitution of the 2N3439/5415 pre-drivers with MJE340/350 and six pairs of output transistors 2SA1216/2SC2292).

Thanks
Zvon
 
#96
Progress

Here are some pics of the final amp modules

\Jens
 

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