Swordfishy/ASPEN FETZILLA power amp - Page 4 - diyAudio
Go Back   Home > Forums > Commercial Sector > Manufacturers > AKSA

AKSA Builders of market proven kits and modules at sound quality to rival the best in high end

Reply
 
Thread Tools Search this Thread
Old 26th June 2011, 01:00 AM   #31
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Let me give some explanation of progress. Paul Bysouth, a local engineer, and Greg Peters, aka SWF on this forum, have been in discussion at some length.

Firstly a decision has been taken to keep this as simple as possible for exceptional performance, no simpler, but certainly not unnecessarily complex, as this would put off early adopters who should be encouraged to build this lovely amp. This has meant power supplies on the same board as the amp, so it can simply be hooked to input/output and trafo, simple mounting onto a large heatsink to permit choice of quiescent currents without thermal stress, and through hole components wherever possible.

Secondly, there is a lot of design input to the switch on/off transients on this amplifier. This is always a vexed issue as people are very leery of seeing their speaker cones excursing across the room a la Back to the Future when the power switch is thrown. Switching spikes are largely caused by settling currents in the input stage; with zero volts at both gate and source at switch on, we have the jfet moving immediately to Idss; in the case of the 2SK170 this is typically 8-12mA and this current viciously turns on the VAS, which responds immediately with a large voltage spike on its drain. The output stage faithfully follows this spike, and voila, we have a spike on the speaker itself.

The correction is effected when the input cap and the fb shunt cap are charged to the set bias voltages; but this takes time. What is worse is that the 1.5uF input cap charges via the 33K bias resistor (time constant 50mS) and the fb shunt cap, fully 1000uF, charges via the source current of the jfet (which initially is at Idss). This charge pulse also prevents immediate current flow through the series fb resistor, further preventing offset settling.

Mitigating circuit conditions are in the power supply, however. LTSpice calculates switching spikes on the assumption that the rails build immediately to the 36V chosen here; in fact the process is more gradual as the filter caps take finite time to charge. This reduces the output spike. With mosfet (DN2530) input device, LTSpice indicates about a 19V spike, yet with the 2SK170 device SWF says the spike is no louder than the sound of the toggle, and the speaker cone hardly moves, indicating much, much less in practice. [EDIT: Paul has just informed me that with the 2SK170 front end device modelling in LTSpice indicates only a 3.6V output spike, less in practice, so all is well. HD] In order to avoid the complexity of a servo, I feel this is quite adequate. If it proves an issue for more fragile single range drivers like Lowthers or tweeters I would suggest using a three position (double throw), three pole power switch, with 1st position off, 2nd position amp on, and the final switch position connecting the hot output of the amplifier to the speaker. Since settling time is less than 100mS, as long as the user is not sniffing white powder all should be well.

I have made subtle changes to the layout. There are vertical overlay lines demarcating the power supplies from the amp module, so that they can be cut off if desired to permit use of off board supplies; smd provision for the ZVP2110G, the mosfet 2W version of the ZVP3110A VAS; RC decouplers on both rails for quieter operation, and rail/ground diodes to offer DC protection to the speaker should only one rail fuse blow. I have configured the output stage so that source is the center terminal, for both Exicon (Profusion) and Hitachi devices. Note that this is NOT the same pinout as the IRF/Vishay hexfet Mosfets, which are Gate Drain Source, left to right. I have also added a very clever CCS/bootstrap hybrid which combines the best of both approaches, courtesy Paul Bysouth, a highly competent local engineer. C1 is a large (20Lx17D), quality axial component, I have selected an Auricap of 1uF. I have split the bias generator into three 4148 diodes and a 200R trimpot to reduce 'accidents'. If quiescent control in hot conditions is poor, the diodes can be relocated on the underside of the pcb, facing the heatsink base. This will offer some thermal feedback and should throttle back the output stage in very hot operating conditions. I have confined max adjustment with 12mA VAS current to about 4.5V. 2.53V gives a calculated 350mA bias current, so this would give us up to just shy of 1A, sufficient for this design - deep Class AB. I'm assuming 0.1R source resistors and 0.7 Siemen tranconductance on the outputs, about the average. In practice this will vary somewhat, but the range of adjustment is quite adequate and ensures that it will be 'safe' to set up. I've done a general tidy up of the pcb, it's looking quite refined now and will, I can assure readers, be very easy to build.

Throughout this design phase Greg and I have moved away from Lineup's current drain approach on the input device using a CCS. While this approach has strong appeal for biasing reasons it slightly reduces PSRR as the very sensitive fb node, the source of the jfet, is coupled via CCS to the negative rail with its half wave rectified pulses due to output stage operation. It also adds at least one other semiconductor. In my experience a CCS is NOT a firewall for noise; it's actually quite sensitive to noise injection. The decision to pass all jfet current through the series fb resistor is logical from a noise POV as the output is one of the best decoupled nodes in any amplifier. This approach does mean that a bias voltage must now be applied to the input jfet gate; but this actually polarises the input capacitor and greatly improves its sonics. Knowing what can befall amps in the field, I always cap couple the inputs of my amps; it's a safe approach and avoids many mishaps.

Greg is presently on holiday in Singapore, Munich and Paris and will be back in Brisbane on 11th July. On arrival he will verify our choice of ZVP2110G and recheck the turn on transient for the Fetzilla.

I have attached the revised board. All my experience with amp design over 15years tells me this will be an extremely good ampifier.


Cheers,

Hugh
Attached Images
File Type: gif FetzillaPCB.gif (45.9 KB, 1253 views)
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com

Last edited by AKSA; 26th June 2011 at 04:40 AM.
  Reply With Quote
Old 26th June 2011, 01:19 AM   #32
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
More orders:

26 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada

Thanks guys, we are rapidly moving towards the magic 100!

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 26th June 2011, 01:43 AM   #33
evette is offline evette  Canada
diyAudio Member
 
Join Date: Jun 2006
Send a message via Yahoo to evette
I would like to know the price before I order.
  Reply With Quote
Old 26th June 2011, 01:52 AM   #34
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Evette,

Mentioned before; $AUD50 a pair, including shipping.

Not cheap, but 2mm FR4, gold immersion, overlay and silkscreen both sides, and competent design, which took a long time.

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 26th June 2011, 02:20 AM   #35
Ejam is offline Ejam  Australia
diyAudio Member
 
Join Date: Aug 2004
Location: Queensland
Default Put me down for a pair of boards

Hey Hugh

Long time between chats, must catch up on the phone. I agree whole heartedly about keep the amp as simple as possible. All too often the engineer in us gets lose and destroys a beautiful thing. As Leo d V said "simplicity is the ultimate sophistication". Anyway, $50 is cheap for quality boards and I have many Hitachi laterals, so why not. Any chance for a final circuit after Greg gets back?

Regards

Anthony Mills
  Reply With Quote
Old 26th June 2011, 03:57 AM   #36
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Hi Anthony,

Nice to hear from you, lines open any time!

You're on the list, now at 28 boards:

26 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 26th June 2011, 04:08 AM   #37
diyAudio Member
 
chlorofille's Avatar
 
Join Date: Jul 2008
Location: Malaysia
Hi Hugh,

I'm up for 2 boards.
  Reply With Quote
Old 26th June 2011, 04:23 AM   #38
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Doing well, 30 boards so far:

2 JoMo Adelaide SA Australia
2 Ian Finch Coffs Harbour NSW Australia
2 Gennaji Hyderabad India
2 Ken Lewis LA USA
2 Ron Russell Kent UK
2 John Kenny Dublin Ireland
2 Francis Morrin Dublin Ireland
2 Vitalica Bucharest Romania
2 Erik Deinum Groningen Netherlands
2 Keith Correa ? India
2 Igor Polyakov California (city?) USA
2 Steve (aka SRH) Whangarei New Zealand
2 George (aka JKC) Calgary Canada
2 Anthony Mills Gold Coast Australia
2 Dinesh (aka Chlorofille) Christchurch New Zealand

Cheers,

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com

Last edited by AKSA; 26th June 2011 at 04:41 AM.
  Reply With Quote
Old 26th June 2011, 04:26 AM   #39
diyAudio Member
 
chlorofille's Avatar
 
Join Date: Jul 2008
Location: Malaysia
Hi Hugh, I'm Dinesh =)
  Reply With Quote
Old 26th June 2011, 10:58 AM   #40
diyAudio Member
 
supernet's Avatar
 
Join Date: Dec 2001
Location: Europe
Nice progress


it is already possible to predict final power (8/4 ohms) or low impedance loads driveability?


Also...what would be recommended heatsink size? Maybe the same like Naksa 100 use for mono channel?
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Aspen Headphone Amp AKSA AKSA 1129 2nd June 2011 05:48 AM
Aspen Maya Vegking AKSA 7 17th May 2011 05:02 AM
DIYaudio to become new Aspen Forum AKSA AKSA 13 2nd May 2011 07:04 AM
Sachiko/aspen slab rmcelroy Full Range 0 9th March 2011 06:34 PM
Aspen HAKSA Group Buy AKSA AKSA 74 16th November 2010 05:17 AM


New To Site? Need Help?

All times are GMT. The time now is 04:51 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2