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Old 5th March 2015, 07:37 PM   #1
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Default Help with Power Amp Design

Hi there,

I'm trying to design a class a/b high power audio amplifier but I'm struggling to keep the distortion low. The simulation measures about 0.2% THD. I've had single-ended designs with far less distortion. What am I doing wrong here? I've tried to design a cascoded differential input stage because I've read that an approach using current mirrors is not DC stable in reality. I'm running each differential amplifier at about 10 mA. How do I calculate C6 / C11 and C2 correctly? The Circuit seems to be more stable without C2... With higher values for C2 I'm getting HF oscillation.
So many questions but I hope to get some thought-provoking impulses.

Greetings
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Old 5th March 2015, 08:09 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by DasSchnitzel View Post
The simulation measures about 0.2% THD. I've tried to design a cascoded differential input stage
The cascode input stage is likely the source of the distortion.
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Old 5th March 2015, 10:18 PM   #3
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Quote:
Originally Posted by DasSchnitzel View Post
Hi there,

I'm trying to design a class a/b high power audio amplifier but I'm struggling to keep the distortion low. The simulation measures about 0.2% THD. I've had single-ended designs with far less distortion. What am I doing wrong here? I've tried to design a cascoded differential input stage because I've read that an approach using current mirrors is not DC stable in reality. I'm running each differential amplifier at about 10 mA. How do I calculate C6 / C11 and C2 correctly? The Circuit seems to be more stable without C2... With higher values for C2 I'm getting HF oscillation.
So many questions but I hope to get some thought-provoking impulses.

Greetings
You are simulating my "spooky" leach input stage (minus the beta enhanced VAS).

Spooky uses the same 12V cascoded IP stage ,BCxxx LTP's .... and does
.002% THD 20K??

32pF miller comp. (spook uses 33pF) ... all is the same. That 10p lead cap does nothing , I'm not using it.

LED CCS's are a better match for the BCxxx + mpsa92/42 IP's (thermally).
Using an LED referenced Hawksford VAS is better for SQ and general
thermal stability.

Below is the basic "spook" , notice how similar it is to yours.
PS- I tried the beta enhancement .. the Hawksford gives the same
THD with better overload behavior.
Edit - actually R55/58 might be giving the bad THD readings (omit them) -
or go the hawksford route.
OS
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Old 6th March 2015, 04:42 AM   #4
jcx is offline jcx  United States
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dual complementary diff pair input is mostly eye candy - not a sure step towards lower distortion

fixed reference bootstrap is also not a great approach - if cascading input diff pair you should bootstrap your cascode ref to follow the AC input signal - you do that fairly well scaling the output V for the bootstrap ref
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Old 6th March 2015, 06:55 AM   #5
knutn is offline knutn  Norway
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If I were you, I would remove the connection between Q29 Collector and Q1 emitter and between Q30 Collector and Q23 emitter.
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Old 6th March 2015, 06:58 AM   #6
jcx is offline jcx  United States
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correction needed
Quote:
Originally Posted by jcx View Post
dual complementary diff pair input is mostly eye candy - not a sure step towards lower distortion

fixed reference cascode is also not a great approach - if cascoding input diff pair you should bootstrap your cascode ref to follow the AC input signal - you do that fairly well scaling the output V for the bootstrap ref
cascoded input is likely not helpful until the rest is capable of single digit ppm distortion performance
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Old 6th March 2015, 06:21 PM   #7
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Thank you for the replies! The Hawksford VAS was a great benefit regarding distortion. I'm now simulating without the current aplifier output stage because it seems to have very low distortion (changes about 0,001% THD with or withour the output stage). I've tried several other changes but I can't get any lower than 0,02 % THD. The cascoded differential input stage doesn't seem to affect the distortion at all. Same THD readings with and without the cascode transistors. Any ideas for improvement? I've also got some nasty offset issues going on (for example attachment shows VAS output plot with no input voltage). Did I do anything wrong there?

Greetings
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File Type: png VAS.png (38.9 KB, 104 views)
File Type: png DC.png (12.0 KB, 98 views)
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Old 7th March 2015, 12:02 PM   #8
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Solved it, R9 was too small. THD now at 0,0017 %
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Old 10th March 2015, 10:27 AM   #9
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Since I changed the VAS transistors with better (especially faster) types, I've got some problems under overload conditions. With the poweramp section connected it gets even worse. The rise of the sinewave turns into a vertical line. Even under normal load conditions the base current of Q14 seems to be distorted while the base current of Q1 is not.
Do you think this is a problem caused by the simulation itself or will this get a real world problem if I use these transistors?
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File Type: png amp2.png (42.5 KB, 43 views)
File Type: png Ibq14.png (15.0 KB, 21 views)
File Type: png output_overload.png (19.3 KB, 11 views)
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Old 10th March 2015, 10:57 AM   #10
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Search for "Baker clamp", you'll probably need something like this.
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