Leach amp troubleshooting

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
N

nattawa

Member
Joined 2010
Joe, I have no idea about that soft start unit in diy audio store. As far as I understand most soft start circuit are designed to deal with the high inrush current associated with high power toroidal transformers, and not every one of them have a delayed speaker relay control, the function that you in fact need. You may want to make sure they fit your needs before purchasing/building one.

I designed and built a soft-starter and speaker relay control unit myself and it worked perfectly in my Pimped Citation Amp project (Leach amp). But it was very much over-engineered and may not suit every one. Mooly's design was a very smart, simple, effective, and elegant one that's why I recommended it.
 
J

joe mobile

Member
Joined 2006
Hi nattawa

please excuse the delay, I have been buisy over the weekend. I searched the forum and took a look at your build of a soft start and speaker control. Your whole build is quite impressive.

I don`t think I really need the speaker delay, because I dont really mind when it plopps on startup, and I dont have the impression it does any harm to the speakers.

My concern was only if the behaviour might point to something that is fundamentally wrong on my amp, but since your replies I` m not worried about that.

The soft start I wanted to install anyways to preserve the psu caps, and I thought maybe it would reduce the plopp as the "shock" on powering up is not so big, but if thats not the case I can live with it.

Thanks again for your kind help!

joe
 
A

Amoralis

Member
Joined 2011
Leach Amp Clone. HOW TO ADJUST OUTOUT VOLTAGE to 0mV ???

Hi guys.
I have a little problem. I made a this amplifier: DELTA AUDIO - Leach Amp Clone.
According to the scheme: http://www.noermoelle-rasmussen.dk/Delta-audio.com/Leach_Clone/10_TRANSISTOR/Leach_ver_5.6.10_sch.pdf

Power supply is +/- 85V. Idle current 125 mA. Power transistors on a single pair MJL4302 & MJL4281 per channel (PCB made ​​for 5 pairs per channel).

How to adjust an output voltage of the amplifier?
I got on one channel plus 35 mV, and another plus 74mV. We need to do 0mV or close to zero. :cool:
 
djk

djk

R.I.P
Joined 2001
Match the diff pairs better, or try reducing the input resistance. The base current flows from one base into the other, the current from the input resistor only provides the difference in the base currents.

A single pair of outputs?

I hope you're driving a 32Ω load.
 
Last edited:
A

Amoralis

Member
Joined 2011
Adjust the output voltage...

djk said:
Match the diff pairs better, or try reducing the input resistance. The base current flows from one base into the other, the current from the input resistor only provides the difference in the base currents.

A single pair of outputs?

I hope you're driving a 32Ω load.
Click to expand...

Yes, at this time i have solder single pair to each chanel of amplifier.
- If I have soldered up all 5 pairs of transistors, the output voltage of the amplifier can be averaged, and become closer to zero?
- Is it possible to align the shift of output voltage by selecting a resistor in the feedback loop?

Thanks.
 
N

nattawa

Member
Joined 2010
If you haven't done so, match the zener diodes up (D1+D2 = D3+D4).

More output pairs will not help "even out" the DC offset. The feedback resister value has nothing to do with the DC offset. The Leach Amp has a unity gain at DC, therefore, changing the feedback resister value will not help.
 
A

Amoralis

Member
Joined 2011
Checking the Zener diode D1 + D2 = D3 + D4;

nattawa said:
If you haven't done so, match the zener diodes up (D1+D2 = D3+D4).

More output pairs will not help "even out" the DC offset. The feedback resister value has nothing to do with the DC offset. The Leach Amp has a unity gain at DC, therefore, changing the feedback resister value will not help.
Click to expand...

Thanks for the recommendation. I checked the parameters Zener diodes. There is a large variance in the group D1 + D2; :)
I will replace them with others, and see the changes.
 
A

Amoralis

Member
Joined 2011
The amplifier circuit here: http://www.noermoelle-rasmussen.dk/..._Clone/10_TRANSISTOR/Leach_ver_5.6.10_sch.pdf

I picked up in pairs diodes D1 + D2 and D3 + D4, with the similar parameters and got the voltage at the bases of the transistors T1 and T6 on +/- 40.7 volts respectively. The output voltage has fallen from 74 millivolts to 50 millivolts.

So, can I shift the voltage at the bases of the transistors T1 and T6 until I get the output voltage is close to zero (when I use 5 pairs of output transistors MJL4281 & MJL4302)?
 
A

Amoralis

Member
Joined 2011
amplifier output voltage

gmphadte said:
50 mV offset at the speaker terminals is safe enough for the speaker. No need to adjust anything. However matching the front end transistors will reduce it further.

Gajanan Phadte
Click to expand...

Thank you, Gajanan Phadte.
"... matching the front end transistors..." - you mean the output transistors (MJL4302/4281 in my case)?
I will try to pick up a pair of output transistors :) then I'll write is what I got with this.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom