Randy Slone "Fig 11.6" amp, modded: will work? - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 21st December 2006, 03:50 PM   #21
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by AndrewT
try to get the resistor current greater than ten times the base current, to minmise variations in transistor CE current modulating the ladder ratio by sucking current from the middle of the ladder.
Yes, this makes sense. How do I know the base current? Do I get this by dividing the VAS CCS current by the hfe of the Vbias transistor? (I guess so.)

Also, your suggestion gives me an upper limit for the resistors. Is there anything I should keep in mind about the lower limit?

Quote:
maybe ESP had the pot on the collector side. You MUST put the pot+resistor between the base and emitter to avoid increase in current if the wiper goes momentarily or permanently open circuit. You set for minimum bias voltage which gives minimum bias voltage and this requires maximum resistance in the BE resistor leg. Neither should the base be connected to the wiper.
This was totally an embarassing mistake of mine.... Rod says clearly that one must set max resistance to begin with. I had confused it. Sorry.

Quote:
nobody fits +-20mF to +-40mF as decoupling. Local decoupling is usually in the range 47uF to 220uF. It's a PSU calculation=smoothing capacitance.
Okay, got it, thanks.

And I think I need to re-word an earlier question. What's the relationship between the time constant of the PSU and the time constant of the NFB's RC?

What do you think of the PCB? Or don't you think much of it at all?
  Reply With Quote
Old 22nd December 2006, 08:22 AM   #22
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi Tc,
Quote:
What's the relationship between the time constant of the PSU and the time constant of the NFB's RC?
input RC<NFB RC<PSU RC.
I have seen and used a recommendation to keep at least half an octave between each pair.
I use input=80 to 100mS, NFB=120 to 140mS, PSU=160 to 200mS (ClassAB) and gives about -1db @ 3 to 4Hz. These give fairly large capacitors compared to most published designs, particularly so if you aim for low impedances.
Quote:
upper limit for the resistors. Is there anything I should keep in mind about the lower limit
find the minimum and maximum voltage across the output stage bases. An EF Darlington will be min =4*0.65V and max = 4*0.65V + Vre. A triplet needs 6times. CFP is different.
Assume the multiplier produces 0.7Vbe for the minimum output and similarly 0.6Vbe for the maximum output, now warm up your calculator or spreadsheet.
__________________
regards Andrew T.
  Reply With Quote
Old 22nd December 2006, 08:45 AM   #23
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Dear AndrewT,

Got the gen, thanks. Will work my way through the numbers and see.
  Reply With Quote
Old 26th December 2006, 08:06 AM   #24
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Default The PCB: v3

Made some changes to the schematic and PCB:

Click the image to open in full size.

Click the image to open in full size.

I've added a first-order RC low-pass filter at the signal input, and reduced the power ground points by one. Now there are just two power ground points, one on either side of the board, plus the signal ground.

I've also added a small ceramic disk capacitor on each supply rail (C14 and C15) in addition to the old electrolytic ones, near the input section of the circuit. I was told that this additional cap can help keep RF out of the circuit and help stability of MOSFET amps.

Made more space for the L1 inductor. I have an inch of length now, and can probably do more than 25 turns of 14SWG copper.

The Q3-Q4 and Q1-Q2 pairs of transistors are now close together and amenable to being tied together to keep close thermal coupling. Also, the two transistors which make up the VAS Darlington pair (Q5 and Q6) are now thermally coupled with a common heatsink. Apparently, as per some passing mention in Randy Slone's book, this is a good idea.

Set different values for the gate resistors for the P-channel and N-channel OPS devices. Earlier, they were all equal.
  Reply With Quote
Old 26th December 2006, 09:22 AM   #25
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
669 is a wonderful (and relatively cheap) transistor.
I am using them to drive both the 140W Leach and 120W Krell.

BUT not for q5. Use 2sc3423 or similar

Any comment on C8 being electrolytic or plastic film? I have a view!

R34 could be raised to match R12 or even higher.

C7*R12<<1.4*C1*R4.
But if the second DC blocking cap in the source unit is also 10uF then the combined RC is about right. This will result in an effective 5uF giving -1db @ about 7Hz.
__________________
regards Andrew T.
  Reply With Quote
Old 26th December 2006, 11:23 AM   #26
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by AndrewT
669 is a wonderful (and relatively cheap) transistor.
I am using them to drive both the 140W Leach and 120W Krell.

BUT not for q5. Use 2sc3423 or similar
I don't know where you get those transistors.... I looked up findchips.com and they turned up a complete blank in all the resellers listed with them.

Quote:
Any comment on C8 being electrolytic or plastic film? I have a view!
Should I aim for something like a polyester? My PCB currently doesn't have space for something like that.

Quote:
R34 could be raised to match R12 or even higher.
Okay. Will note it.

Quote:
C7*R12<<1.4*C1*R4.
But if the second DC blocking cap in the source unit is also 10uF then the combined RC is about right. This will result in an effective 5uF giving -1db @ about 7Hz.
Which is the second DC blocking cap? I can't see any other cap of 10uF. Do you mean the (presumed) output cap at the output of the CD player or preamp or whatever which is upstream of this power amp?

How do I calculate the HF F3 for my C1+C2+R34+R4 combo?
  Reply With Quote
Old 26th December 2006, 12:03 PM   #27
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
yes, DC blocking cap in the immediately preceeding buffered source component (pre-amp or CD player etc.).
Ignoring the small value of R34
then F-3db=1/2/Pi/R/C
R=R4
C=C1 & C(pre-amp), C=C1*Cp/[C1+Cp]
10uF & 10uF =5uF
10uF & 4u7F=3u2F
10uF & 2u2F=1u8F

Notice how the bass cut-off gets worse as the preamp DC blocker moves to a lower value.

R4=50k, C1=10uF, Cpre=2u2F gives a good compromise.
or swap C1 & Cpre, same result. Buy a good 10uF pp and fit it in the pre, then all your poweramps need just 2u2F pp.

HF F-3db uses R34 & C2
150r & 100p gives -1db @ 5MHz (a bit high don't you think?).
330r & 1nF gives -1db @ 240kHz. (even this is a bit high, but if Rs>1k0 then the story is completely different).

2sc3423 is the complement to 2sa1360. or try complements to 1209, 1210, 1370, 1371, 1380, 1403, 1405, 1406, 1407, 1476, 1477, 1478, 1538, 1539, 1540, 1697, 1707, 1777, 1875, most of these are more expensive. But you should be able to find a number of low Cob alternatives that are suitably voltage rated.
__________________
regards Andrew T.
  Reply With Quote
Old 27th December 2006, 05:42 AM   #28
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by AndrewT
then F-3db=1/2/Pi/R/C
Understood your calculations, thanks again.

Quote:
Notice how the bass cut-off gets worse as the preamp DC blocker moves to a lower value.
So, basically if I need to be preamp-independent in my LF cut-off, I'll need to spend richly on large C1.

Quote:
HF F-3db uses R34 & C2
150r & 100p gives -1db @ 5MHz (a bit high don't you think?).
330r & 1nF gives -1db @ 240kHz. (even this is a bit high, but if Rs>1k0 then the story is completely different).
I think I'll switch R34 and C2 to approximately the values you've mentioned, between 300 and 350 for R34 and about 1nF for C2. I agree that 5MHz is very high.... this high cutoff was considered adequate by Randy Slone in his book to protect MOSFET OPS against oscillations, but I thought that there's no audible harm in playing safe and bringing the cutoff down by a decade or so.

Will a ceramic disc do for C2, or should I opt for more expensive caps?

Quote:
2sc3423 is the complement to 2sa1360. or try complements to 1209, 1210, 1370, 1371, 1380, 1403, 1405, 1406, 1407, 1476, 1477, 1478, 1538, 1539, 1540, 1697, 1707, 1777, 1875, most of these are more expensive.
Okay, will search. Where do you usually find these transistors being sold? I used to look up Digikey and one or two others, now I'll look elsewhere.

Also, sorry to admit this, but I still don't know what Cob is. I didn't find any Cob mentioned in the one or two transistor specs I studied.
  Reply With Quote
Old 27th December 2006, 06:48 AM   #29
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by AndrewT
2sc3423 is the complement to 2sa1360. or try complements to 1209, 1210, 1370, 1371, 1380, 1403, 1405, 1406, 1407, 1476, 1477, 1478, 1538, 1539, 1540, 1697, 1707, 1777, 1875...
I ran each of these partnumbers through findchips.com. About three of these are available at one or two retailers, the others are not available with even one retailer. Can you suggest any specific retailers I should try?
  Reply With Quote
Old 27th December 2006, 07:10 AM   #30
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by AndrewT
2sc3423 is the complement to 2sa1360.
Found 2SA1360 on MCM (www.mcminone.com). It's very inexpensive, at about forty cents each. 2SC3423 is more expensive, at a shade over a dollar each. Both are Toshiba devices. 2SA1370 and 1371 are also available, at prices broadly in the same range as 1360. I've stopped searching for the remaining parts for now... it seems a sufficient portion of your list is available from here.

Strange how these are so hard to find through the findchips.com resellers.

Can you please explain what will be the audible or stability effect if I use these transistors instead of the 649/669 devices? And why is the swap needed only for Q5?
  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
Randy Slone's "Fig 11.4" (Self's "Blameless"?): PCB layout tcpip Solid State 128 23rd September 2013 04:45 PM
Randy Slone 6.6 design py Solid State 8 20th July 2011 02:06 PM
Project 11.1 from Slone "High-Power Amplifier" Book Karl71 Solid State 46 6th October 2008 03:47 AM
Randy Slone JDDCo Solid State 6 22nd September 2006 02:57 AM
OPTIMOS-Amp from Randy Slone wopo Solid State 1 12th July 2004 08:18 PM


New To Site? Need Help?

All times are GMT. The time now is 01:27 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