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panson_hk
diyAudio Member

Join Date: Aug 2005
Quote:
 Originally Posted by PeterLeest What is the bias current to get the optimum result ?
Peter,

I have some more data which should be able to answer your question. The output waveform (1W/8Ohm) and THD residual are monitored for various bias levels. FFT of the residuals are also shown in the figure.

You can see very large spikes of cross-over distortion for zero bias. When the bias current is 50 mA per device, the cross-over distortion is reduced substantially (notice the vertical scale). At 100 mA bias level, we see no more cross-over distortion. The distortion is mainly contributed by 3rd harmonic.

I use 0.22 Ohm emitter resistors (Re) for the output devices. The voltage drop is 22 mV. Optimum Vq (2 Re drop) for EF is 42 mV, as given in Self's book. Here is 44 mV, more less the same. This is the optimum value.

Panson
Attached Images
 Cross-Over Distortion zero bias.JPG (103.7 KB, 2402 views) Cross-Over Distortion bias 50mA.JPG (111.9 KB, 2238 views) Cross-Over Distortion bias 100mA.JPG (120.3 KB, 2192 views) FFT of residual vs bias level.JPG (93.1 KB, 2160 views)

 5th November 2009, 07:48 PM #12 traw   diyAudio Member   Join Date: Sep 2005 Looks like have drivers... mje15030/31? Quick question, how does one know or determine right size resistor that is often strung betweeen the emitter's of the top and bottom driver, before it feeds the gate resistors? What happens if not have in place?... sorry for off topic, but cannot find much info about creating driver. Wish to run 3-4 parallel mosfets. In an1645 doc they show a 100ohm but tied to output rather than to other driver emitter.
panson_hk
diyAudio Member

Join Date: Aug 2005
Quote:
 Originally Posted by traw Looks like have drivers... mje15030/31? Quick question, how does one know or determine right size resistor that is often strung betweeen the emitter's of the top and bottom driver, before it feeds the gate resistors? What happens if not have in place?... sorry for off topic, but cannot find much info about creating driver. Wish to run 3-4 parallel mosfets. In an1645 doc they show a 100ohm but tied to output rather than to other driver emitter.
Yes, the drivers are 15030/31.

That resistor value can be determined by how much standing is going to pass the driver transistors. I have 1.226 V across 180 Ohm. Hence, the driver bias current is 6.8 mA. The voltage drop across this resistor together with the emitter resistors of output devices sets the output bias current. You simply can't without it.

You can find some information from this link regarding Leach Amp The Leach Amp - Output Stage. Look for R34, 35, 36.

panson_hk
diyAudio Member

Join Date: Aug 2005
Output resistance

I measured the output resistance. Output circuit output voltage was set to 10 Vrms, 1 kHz. The AP recorded the output level without loading from 10 Hz to 50 kHz. The output level was then measured again for 8 Ohms and 4 Ohms load. The amplifier output resistance is estimated by using the measured data. The measured was repeated for output level set to 20 V rms, unloaded.

It can be seen that the output resistance in audio band is below 20 mOhms. Damping factor, ignoring cable effect, is > 400 for 8 Ohms.
Attached Images
 output resistance.JPG (77.8 KB, 2063 views)

Last edited by panson_hk; 13th November 2009 at 07:53 AM.

klewis
diyAudio Member

Join Date: Sep 2007
Quote:
 Originally Posted by panson_hk Peter, I have some more data which should be able to answer your question. The output waveform (1W/8Ohm) and THD residual are monitored for various bias levels. FFT of the residuals are also shown in the figure. You can see very large spikes of cross-over distortion for zero bias. When the bias current is 50 mA per device, the cross-over distortion is reduced substantially (notice the vertical scale). At 100 mA bias level, we see no more cross-over distortion. The distortion is mainly contributed by 3rd harmonic. I use 0.22 Ohm emitter resistors (Re) for the output devices. The voltage drop is 22 mV. Optimum Vq (2 Re drop) for EF is 42 mV, as given in Self's book. Here is 44 mV, more less the same. This is the optimum value. Panson

klewis
diyAudio Member

Join Date: Sep 2007
Quote:
 Originally Posted by panson_hk Peter, I have some more data which should be able to answer your question. The output waveform (1W/8Ohm) and THD residual are monitored for various bias levels. FFT of the residuals are also shown in the figure. You can see very large spikes of cross-over distortion for zero bias. When the bias current is 50 mA per device, the cross-over distortion is reduced substantially (notice the vertical scale). At 100 mA bias level, we see no more cross-over distortion. The distortion is mainly contributed by 3rd harmonic. I use 0.22 Ohm emitter resistors (Re) for the output devices. The voltage drop is 22 mV. Optimum Vq (2 Re drop) for EF is 42 mV, as given in Self's book. Here is 44 mV, more less the same. This is the optimum value. Panson
Oops, let me try again...

Panson, I could really use your help. My amp is running, sounds great, makes nice sine and square waves at 1k, 10k and 20k hz. So, I can't for the life of me measure the voltage drop across Re. Here's how I'm set up for the measurement: power on, input shorted, DMM probes set at emmitter of pnp and npn output transisitors, reading is 0v. If I put the probe at the emmitter of either transistor and ground, I get .006v, at the base of either transistor I get .475v.

Any thoughts?

Thanks,

Ken

panson_hk
diyAudio Member

Join Date: Aug 2005
Quote:
 Originally Posted by klewis Oops, let me try again... Panson, I could really use your help. My amp is running, sounds great, makes nice sine and square waves at 1k, 10k and 20k hz. So, I can't for the life of me measure the voltage drop across Re. Here's how I'm set up for the measurement: power on, input shorted, DMM probes set at emmitter of pnp and npn output transisitors, reading is 0v. If I put the probe at the emmitter of either transistor and ground, I get .006v, at the base of either transistor I get .475v. Any thoughts? Thanks, Ken
Ken,

Are you using Leah triple output stage and Vbe multiplier? Based on your values, I believe the output transistors do not turn on. Can you measure base-to-emitter voltage? the base to emitter should be above 0.5 V when the device is active. What is the bias voltage generated by the Vbe multiplier? Do you have a potentiometer for bias level trimming? When you measured, was the output connected to a speaker, float or a dummy load (8Ohms)?

Panson

 15th November 2009, 01:59 PM #18 klewis   diyAudio Member     Join Date: Sep 2007 Panson, In the process of cleaning up my schematic so that I could put measure voltages on it for posting, I think I found an error. I will fix the error and see what happens. Ken
AndrewT
diyAudio Member

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by klewis DMM probes set at emmitter of pnp and npn output transisitors, reading is 0v. If I put the probe at the emmitter of either transistor and ground, I get .006v, at the base of either transistor I get .475v.
the voltage acros one Vbe should be +ve and the other should be -ve.
You say both are 0.475Vdc is either -ve or both +ve.

Measure from emitter to emitter of the output pair.
Measure from base to base of the output pair.
Measure from base to base of the driver pair.
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jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
 Originally Posted by klewis Oops, let me try again... Panson, I could really use your help. My amp is running, sounds great, makes nice sine and square waves at 1k, 10k and 20k hz. So, I can't for the life of me measure the voltage drop across Re. Here's how I'm set up for the measurement: power on, input shorted, DMM probes set at emmitter of pnp and npn output transisitors, reading is 0v. If I put the probe at the emmitter of either transistor and ground, I get .006v, at the base of either transistor I get .475v. Any thoughts? Thanks, Ken
Assuming you measure the voltage across the Re without signal, then your measurement shows underbiasing. Vbe < 0.6V so no bias current so no drop across Re. Increase the bias until you see Vre rising, until you have the desired bias current.

jd
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