need help with leach amplifier

[sng001]

Originally Posted by elektrogeek

Its at 25v because I was afraid that something might go wrong, since the output would go out, but i shall raise the supply voltage. Also, on r21 and r22 when you power it up the dc voltage on r21 is slowly decreasing and the dc voltage on r22 is slowly rising. Then when the "phenomenon" occurs, there is quick drop of voltage on r21 and a quick increase on r22. I noticed that if the two voltages were added together it would equal the supply voltage.

Thanks

Andy

Hi Andy,

I used Dr Leach's method to test the Amp at full supply voltage (52V). It did not blow up anything expensive as the output transistors were not connected.

I have attached an FAQ here from Dr Leach's site:
The Leach Amp FAQ

"I do not need the 120 W power and I am going to remove transistors Q20 and Q21. Should I change any other elements, e.g. R36, R37/R38, or R41/R42? What about connecting C19 and C20 to R37 and R38?
Removing 1/2 the output transistors will not reduce the output power. It reduces the maximum output current by a factor of 1/2. For an 8 ohm load, the power output would not change. For lower load impedances, the protection circuits would kick in at a lower output current than with all 4 output transistors. If you want to operate the amplifier with 1/2 the output transistors, Q18 and Q19 are the ones to omit. You can also omit R37, R38, R41, R42, R45, and R46. These changes will reduce the maximum output current by a factor of 1/2. If you wish to decrease the output power, you must reduce the rail voltages. I do not recommend reducing them to less than +50 V and -50 V."

Good luck with your amp.

[AndrewT]

Hi,
this reminds me of when I suggested that we should stop specifying amplifiers by Power.
Instead I said we should specify by output voltage and output current with a recommended load impedance.

eg, is a 22Vac 5Aac amplifier significantly different from a 24Vac 4.5Aac amplifier?
In Power rating it looks like there is an extra 11.5W available from the 24Vac amp.
I can and I'm pretty sure everyone else can see that there is actually very little difference between the amps. This avoids all the confusion and bragging rights of using exaggerated, or even truthful Power ratings.

Now if one were to compare amp outputs of:
22Vac 10Aac
to
24Vac 4.5Aac

I think we can all see there is a significant difference.

The 22Vac is designed to drive lower impedance loads than the 24Vac amplifier.

[nattawa]

Originally Posted by elektrogeek

......when you power it up the dc voltage on r21 is slowly decreasing and the dc voltage on r22 is slowly rising. Then when the "phenomenon" occurs, there is quick drop of voltage on r21 and a quick increase on r22. I noticed that if the two voltages were added together it would equal the supply voltage.

Andy, the input stage of a Leach amp should work just fine in terms of DC setup at your supply voltage. The strong degeneration of the diff-pairs insures a stable and symetric DC status of the input stage even before the zener diodes begin to kick in. At +/-25V supply you should get about 0.79mA per transister (Q1 - Q4) by my quick calculation.

Therefore, with +/-25V supply, at no signal input, you should be expecting the following stable DC voltages (in the neighbourhood of):

voltage across C2/C3 = 19.8V
voltage across R11/R12 = 0.94V
voltage across R21/R22 = 0.026V

You should never see voltages greater than 0.6V across R21/R22 as it's supposed to be clamped by the emitter P/N junction of the Q8/Q9.

You probably need to double-check your board thoroughly for any mistakes/bad-parts/wrong-parts.

[elektrogeek]

Hi, I think what my problem is, is my pcb. I made it myself with the provided foil pattern. As I look at it, I can see some spots that didnt show up when I removed the press 'n' peel sheet. Kinda embarassing. But, I want to order a set from Dr. leach. But I have emailed him like 4 times and he hasn't responded.

[Mooly]

Originally Posted by elektrogeek

Hi, I think what my problem is, is my pcb. I made it myself with the provided foil pattern. As I look at it, I can see some spots that didnt show up when I removed the press 'n' peel sheet. Kinda embarassing. But, I want to order a set from Dr. leach. But I have emailed him like 4 times and he hasn't responded.

Is it not fixable ?... at least for you to prove it all works. Any shorts can be cut with a craft knife, any opens can be linked.