Leach Low Tim Blew Outputs

[gmphadte]

I assembled a stereo Leach low TIM amp and tested it. Everything was fine.

Then I assembled a Lightspeed Attenuator and a Pedja Rogic buffer and all tested to work fine individually. I then assembled them in the order below.

1)Lightspeed with motorized RF remote control. The remote has two more channels. One is used for input selector relay to switch between two inputs, and the other was used to switch On/Off the lightspedd 5Volts. I found that this cannot be done as the lightspeed goes into high resistance and the amp hisses.

2) The Pedja Rogic buffer- I think it attenuates the signal. I have to check on this.

3)Leach Low TIM amp

4)Hafler speaker protection.

The motorsed pot was cracking and I had removed the on board protection on the Leach amps.

One of the amps blew the output transistors(MJ15003/04)as I had removed the on-board protection, either due to the switching of the 5Volts DC to lightspeed or the cracking motorised pot. Both these were poping the speakers.

I have decided to either do away with the 5V switch or fix a filter cap after the switch.

Cracking pot issue will presently be handled by replacing or repairing the worn out pot track, may be by shifting the wiper contact. The whole motorized pot will not be replaced as it is costly and also is not available. Any how, that pot is a POS.

I pulled it out from a cheap Sony stereo. I have another Sony with a better quality pot. This pot will be used as a last resort.

That is for the facts.

I find that the Low TIM needs lots of juice at the input. Can anybody point me to a transistorized preamp that works at 40+ dual supply.

Gajanan Phadte

[osscar]

I also have noticed that Leach amp needs buffer or preamp . With a passive preamp it seems that the input impedance is reduced - so you have the feeling that Leach had insufficient gain. Chipamps works very well with passive preamp (input impedance of about the same as Leach - 20Kohm).
I think so, someone smarter than me will explain it in details.

[gmphadte]

The motorised pot I connected had one end common to both tracks and so cannot be connected in the needed configuration for the lightspeed attenuator. I tried to use it by shorting wiper to opposite sides, but as the pot has audio taper(IMO), does not work as the values don't vary in inverse-proportion(sort of) manner.

I changed the pot to a dual linear and the things r fine.

But it(Low TIM amp) surely needs some more gain.

Gajanan Phadte

[AndrewT]

Have you downloaded the Lo Tim paper?
Leach Amp Plans - Part 1

You will find all the information you need to build it properly and sufficient background to enable most builders to change the gain from 21 times (+26dB) and to change the input impedance from 20k.

[gmphadte]

AndrewT

thanks for the information.

I will keep the gain as intended and add a preamp at the input so that the main aim i.e. low TIM (parameter) is not affected.

Gajanan Phadte