Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

My attempts at a design of a 3 stage amplifier
My attempts at a design of a 3 stage amplifier
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th July 2019, 04:03 PM   #611
edbarx is offline edbarx  Malta
diyAudio Member
 
Join Date: May 2018
The earth connection has been previously secured with an insulated bolt to the bottom of the amplifier box. This has been exchanged with a longer stainless steel bolt. In the original setup, the paint at the contact area has not been removed. The only contact is from the sides of the drilled hole in the metal. To improve the contact I removed some paint, butn found the paint is quite tough to remove. I will uncrew the bolt and remove more paint to obtain a better earth connection to the amplifier box. Aside from regulations, this connection can be a life saver, and I haven't yet given up to living on this planet.

The input circuitry, now shares the same ground as power. Using a separating 10R resistor resulted in more pronouced issues. I remember from reading Douglas Self that an input cascode which is biased from a voltage devider outside the input stage, may suffer hum issues: Self remarks nobody yet understands why.

Simulating for the current source feeding the input stage, I found it is modulated by the input signals and power supply ripple. I am associating the little remaining hum/buzz with a current source that deviates from having an infinite output impedance. I also found the bias supplying the current sources can be further made independent of power supply modulation.

The Huge Toroidal Transformer:
I did several tests to determine whether the huge toroidal transformer, is injecting hum/buzz signals in the input stage via magnetic induction. All tests indicate this is not the case, or if it is, the effect must be very minimal.
  Reply With Quote
Old 20th July 2019, 04:41 PM   #612
edbarx is offline edbarx  Malta
diyAudio Member
 
Join Date: May 2018
Replacing the input stage's current source with an ideal current source, resulted in no modulation and no ripple fed into the VAS. This confirms I am on the right track.
  Reply With Quote
Old 20th July 2019, 08:27 PM   #613
mjona is offline mjona  New Zealand
diyAudio Member
 
Join Date: Jul 2007
Location: Upper Hutt
You need to review the values of R9 and R30 in your current mirror.

For more information run a search "comments on audio power amplifier design handbook by douglas self" and read section 3.

There is an example you can look up on this site - the popular Explendid amplifier which demonstrates the point. This does not have a cascode stage in the LTP however there is a Technics high power amplifier that does which uses 680R emitter resistors.

Last edited by mjona; 20th July 2019 at 08:32 PM.
  Reply With Quote
Old Yesterday, 04:45 PM   #614
edbarx is offline edbarx  Malta
diyAudio Member
 
Join Date: May 2018
Rail decoupling filters for the input stage are being confirmed to improve the performance of the input stage. Removing them for testing purposes, resulted in increased hum issues.

The 10R ground splitting resistor is used to reduce earth loop currents due to different earth potentials. However, I am still mulling where is the best place for this in my amplifier schematic. I would like to keep the input and output with a continous ground, and insert such a resistor just before the input. This way, unwanted ground currents from an external music player should be reduced.

Using a cascode for the input stage current mirror reduced the ripple to 55uV peak to peak. The modulation of the current mirror's output was still modulated by the ripple, but to a much lesser extent.

This may be naive on my part:
I am after ZERO modulation of the input current mirror's output current.
  Reply With Quote
Old Yesterday, 05:56 PM   #615
mjona is offline mjona  New Zealand
diyAudio Member
 
Join Date: Jul 2007
Location: Upper Hutt
Quote:
Originally Posted by edbarx View Post
Rail decoupling filters for the input stage are being confirmed to improve the performance of the input stage. Removing them for testing purposes, resulted in increased hum issues.

The 10R ground splitting resistor is used to reduce earth loop currents due to different earth potentials. However, I am still mulling where is the best place for this in my amplifier schematic. I would like to keep the input and output with a continous ground, and insert such a resistor just before the input. This way, unwanted ground currents from an external music player should be reduced.

Using a cascode for the input stage current mirror reduced the ripple to 55uV peak to peak. The modulation of the current mirror's output was still modulated by the ripple, but to a much lesser extent.

This may be naive on my part:
I am after ZERO modulation of the input current mirror's output current.
You have not commented on the point I made about increasing the emitter resistor values for the current mirror transistors.

That seems to suggest you think you know it all in which case why are you asking this question.
  Reply With Quote
Old Yesterday, 09:09 PM   #616
edbarx is offline edbarx  Malta
diyAudio Member
 
Join Date: May 2018
Quote:
Originally Posted by mjona View Post
You have not commented on the point I made about increasing the emitter resistor values for the current mirror transistors.

That seems to suggest you think you know it all in which case why are you asking this question.
Had I known it all, this project would have been completed months ago. But, alas, I cannot get rid of this hum. Regarding increasing the emitter resistors of the current mirror, I simulated increasing the resistance to 330R, but did not get simulation results that suggest this will correct the output hum. The only changes that suggest the hum will be reduced are using rail decoupling and a cascode for the input stage current mirror.
  Reply With Quote
Old Today, 06:39 AM   #617
mjona is offline mjona  New Zealand
diyAudio Member
 
Join Date: Jul 2007
Location: Upper Hutt
If you run the Tian plot for your circuit you will see the gain at 1Hz is less than 30 dB. That is less than the 35 dB of closed loop gain you are expecting this to deliver.

A simulation I posted last year had 76 dB of gain at 1Hz and 25.7 dB of closed loop gain leaving roughly 50 dB to deal with distortion and hum.

I suggest you go back over your files to the point prior to incorporating the circuit protection where some resistor values in the driver stage might have changes to accommodate that addition - then run a Tian test on one of these.
  Reply With Quote

Reply


My attempts at a design of a 3 stage amplifierHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Phono stage design considerations part 1: choosing 1st stage tube Joshua_G Tubes / Valves 215 26th August 2018 05:30 PM
Amplifier design: how to connect a stage with the next one? Bricolo Solid State 11 10th August 2018 07:34 AM
Pre-amplifier circuit not working despite attempts to fix it Rudi1441 Analog Line Level 5 18th July 2016 02:35 PM
LF Line Level Stage design to preceed class D amplifier undefinedza Analog Line Level 13 18th September 2014 08:26 AM
how to design mosfet amplifier stage? pzung Solid State 2 22nd January 2010 06:58 PM


New To Site? Need Help?

All times are GMT. The time now is 09:27 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki