Another Leach amp offset problem!

[supernet]

I have finished my Leach amp yesterday using BrianGT boards and Toshiba SA1943 and SC5200 output transistors.

When I tested my circuit boards without output transistors they works fine and have about 15 mV offset (tested at +/- 30 V), but when I finished amplifier (+/- 60 V) and set the bias I get about 700 mV!!!!! offset in right channel and 600 mV in the left. Actualy amplifier is working but I get also some hum noise.

Please help me!

Supernet

[BrianGT]

Did you match any of the components in it? I matched my input resistors and Q1-Q4, and I get 20mV and 30mV offset at full power.

--
Brian

[peranders]

Your problems are located at the input stage (98% sure). The offset with SHORTED input DC-wise) will generate 10-50 mV if the DC-gain is 1. If the DC gain is 20-30 the offset will be times the gain, not in your case I suppose.

Mr Leach way of fixing current for the input transistors is not very common. Since your amp works at lower voltage I think your problem is purely component values, maybe you need a tune up?

Try to measure all currents/voltages around the input stage and post the results. Maybe we can sort your problems out?

[mlloyd1]

Are you sure this "offset" is DC, not some average value due to oscillations? What does the 'scope say? Or is the output RLC warm?

And how loud is the hum?

Originally posted by supernet
I have finished my Leach amp yesterday using BrianGT boards and Toshiba SA1943 and SC5200 output transistors.

When I tested my circuit boards without output transistors they works fine and have about 15 mV offset (tested at +/- 30 V), but when I finished amplifier (+/- 60 V) and set the bias I get about 700 mV!!!!! offset in right channel and 600 mV in the left. Actualy amplifier is working but I get also some hum noise.

Please help me!

Supernet

[supernet]

Thanks for reply!

I matched only one channel but there is no significant difference 600 mV or 700 mV (unmatched). When I tested the circuit boards with osciloscope there was nice, phase correct sin signal, but my bench supply goes only up to+-30V.

I am thinking that maybe something is wrong with power supply, because I get some hum (not loud) in the 50Hz bass region.(maybe superimposed mains noise).

Left channel is working, right not.I hear hum from both.Transformer have 2×42V secondaries per channel which give about +- 61 V (where is voltage drop from two bridges???)

Hope this helps!
My power supply:

[BeanZ]

If your RMS voltage from the wall is about 122V and the transformer has 115V primaires. Your output DC voltage will be slightly above 61V with the diode drops. As for the offset problem, see what the offset voltage is with a load on the output. If there is no load connected, there is sometimes a larger DC offset measured because the source impedance of the amp is small compared to the huge impedance of the test equipment. When a load is present, the offset should be nearly zero.

BeanZ

[supernet]

Actually I got 230V mains and transformer got 2×42V without load.

If I connect speaker to amplifier then offset is about -0.420V on positive terminal.

[Steve Eddy]

Originally posted by BeanZ
As for the offset problem, see what the offset voltage is with a load on the output. If there is no load connected, there is sometimes a larger DC offset measured because the source impedance of the amp is small compared to the huge impedance of the test equipment. When a load is present, the offset should be nearly zero.

It's the <b>unloaded</b> offset voltage that counts. That's the voltage impressed across the voice coil of your loudspeaker which determines how much DC current will flow through it and how much of an offset of the speaker cone it will produce, which is what keeping <b>open circuit</b> offset voltage as low as possible is all about.

Of course you'll measure a lower voltage with a load attached. That's because you're measuring the voltage drop across the resistance of the load due to the current flowing through it. In a loudspeaker, that current produces a magnetic field which causes the cone to become offset from its point of equilibrium, resulting in increased distortion from the loudspeaker.

If one doesn't care about this added distortion, that's fine. But if your goal is to keep it to a minimum, you want to make sure that the unloaded offset voltage is as low as possible.

se

[BenY]

1)You should check the zener diodes in the differential input stage,Make sure the negative and positive sets are matched,It`s much more important to match the zeners then the transistors.
2)as far as noise you should tap the power supply ground dead center between the main supply caps.

You`re welcome...

Yuval.

[mikek]

Originally posted by supernet
I have finished my Leach amp yesterday using BrianGT boards and Toshiba SA1943 and SC5200 output transistors.

When I tested my circuit boards without output transistors they works fine and have about 15 mV offset (tested at +/- 30 V), but when I finished amplifier (+/- 60 V) and set the bias I get about 700 mV!!!!! offset in right channel and 600 mV in the left. Actualy amplifier is working but I get also some hum noise.

Please help me!

Supernet

As mloyd has suggested, your problem might be caused by the amp. oscillating......this causes an appreciable voltage drop across the amp.'s output inductor, and is expressed as a persistent offset at the output........