DIGI-125 Kit Amplifier Module - Page 4 - 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 10th April 2008, 11:26 AM   #31
diyAudio Member
 
Greg Erskine's Avatar
 
Join Date: Jan 2002
Location: Sydney/Australia
Quote:
Originally posted by Muff
The digi125 is a class A amplifier, confirmed when I contacted the designer Graham Dicker.
Hi Muff,

The standard Digi125 is definitely not a class A amplifier. With only 2 diodes for bias it is lucky to get warm.

Are you referring to a different or modified amp?

regards
__________________
Greg Erskine
  Reply With Quote
Old 10th April 2008, 02:35 PM   #32
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
"He sent me instructions with the last batch of boards I purchased last year, for a 200watt version which uses the MJ15003/004 devices, however in this form the Amp is in class B operation."

Aussie watts?

I might believe it with a huge supply for a very, very, short time before the magic smoke is let out.

The Peavey CS400s used three pair on 57V with a healthy transformer for 200W/4R.

I built some Pass A40 boards driving three pair of MJ15015/16 on 57V that still run (after almost 30 years). The amp had two small transformers (~200VA) and did 200W/4R, with two 15,000F per channel.

The A40 boards are still available for $6 from AudioXpress.
__________________
Candidates for the Darwin Award should not read this author.
  Reply With Quote
Old 10th April 2008, 04:15 PM   #33
Muff is offline Muff  Australia
diyAudio Member
 
Join Date: Mar 2008
Send a message via AIM to Muff Send a message via MSN to Muff Send a message via Yahoo to Muff
Quote:
Originally posted by Greg Erskine


Hi Muff,

The standard Digi125 is definitely not a class A amplifier. With only 2 diodes for bias it is lucky to get warm.

Are you referring to a different or modified amp?

regards
Well now I`m a little confused I have to confess, I thought in class b operation, there is no forward bias, therefore no quiescent current flows i.e. the output devices are not turned on, in the digi125 the quiescent current is around 40ma provided by the two diodes.
  Reply With Quote
Old 10th April 2008, 04:48 PM   #34
diyAudio Member
 
Join Date: Feb 2008
Quote:
in the digi125 the quiescent current is around 40ma provided by the two diodes.
No, not!

The diodes are providing 0.7+0.7=1.4V bias. The EF stage (drivers+ouputs) needs 2.2...2.4V.
So the bias current in the Digi125 is almost zero.
  Reply With Quote
Old 10th April 2008, 08:02 PM   #35
Bone is offline Bone  United Kingdom
diyAudio Member
 
Join Date: Jun 2004
Location: Dorset, UK
Not all diodes drop the standard 0.7v. This value is only a convenience to make calculations easier.
The volt drop or Vf is dependent on the current flowing. Its not like a resistor; the Vf/If graph is not linear and Vf is not a constant for all values of If.
The 1N914 for instance has a Vf of 1v at 10mA forward current whereas the 1N400x series have a Vf of 0.7v at an If of 10mA and reaches 1v Vf at 1A If! Different types of diodes will have less or greater forward voltages. Similarly the transistor Base Emitter diode Vf is not a constant 0.7v for all types of transistors! This again depends upon the current flowing through the BE diode and can be lower than the assumed 0.7v. Actual values can be obtained from datasheets.

So long as the forward current is a constant the Vf will depend upon this current but also the ambient temperature which is why diodes are used to give a small temperature compensation.

I hope I have explained this OK.

Tony.
  Reply With Quote
Old 11th April 2008, 06:57 AM   #36
Muff is offline Muff  Australia
diyAudio Member
 
Join Date: Mar 2008
Send a message via AIM to Muff Send a message via MSN to Muff Send a message via Yahoo to Muff
Quote:
Originally posted by djk
"He sent me instructions with the last batch of boards I purchased last year, for a 200watt version which uses the MJ15003/004 devices, however in this form the Amp is in class B operation."

Aussie watts?

I might believe it with a huge supply for a very, very, short time before the magic smoke is let out.

The Peavey CS400s used three pair on 57V with a healthy transformer for 200W/4R.

I built some Pass A40 boards driving three pair of MJ15015/16 on 57V that still run (after almost 30 years). The amp had two small transformers (~200VA) and did 200W/4R, with two 15,000F per channel.

The A40 boards are still available for $6 from AudioXpress.

Ok here are the instructions I was given for the 200watt version by the author, I am quoting verbatim.

================================================

Digi-200 modifications

The secret to modifying the digi125 amplifier to produce 200 watts RMS into 2 ohms lies in some basic improvements to the thermal design of the module.
Normally to produce levels of this amount one would expect to find two pairs of MJ15003/004 devices in the output stage, the main reason being to ensure the 50A (Safe Operating Area) of the output devices are not exceeded. We are able to obtain 200 watts from a pair of devices and still remain inside the 50A by using two design techniques. (1) by using low power supply rails with a low load impedance. (2) by using good design practice with regard for the thermal consideration.

Instead of mounting the module to a small aluminum bracket and then to a heatsink we now mount the PCB and output transistors directly to a heatsink, this lowers the thermal resistance to air and improves the heatsink efficiency. Thus enabling us to use only one pair of output devices. A large heatsink with a low thermal resistance such as a DSE H3472 or a Jaycar MF-21-1F-75 with a thermal resistance to air of .6 deg/c/watt is recommended.

The changes required are to insert a wire link in the PCB in place of R7 and R8 (As the output devices are not turned on i.e. they are in class B no quiescent current flows . This also improves the damping factor ( lowers the output impedance of the output stage.).

Q6 is changed for a MJ15003 and Q7 for a MJ15004. R6 must be a proper 1/2 watt resistor.

For increased voltage gain (av=50 instead of av=20) R4 can be reduced in value to 2K2.

In the schematic diagram Q4 should be a BC556 R3 and R2 should be 4K7.

================================================

At the end of the day I like what sounds good, I was in hifi retail for a few years and saw some nice equipment, in those years, but I would not part with my digi125 based amp for anything.
I built it 20 years ago and to date has never had any sort of failure, things only go pop and fizz when the wrong sort of load is driven or the amp has poor thermal characteristics or a combination of the two.
My implementation of the digi125 is four modules in bridged pairs,
for a two channel output. The power supply x4 are based on a 25-0-25 volt 300VA toroid and a bank of 8 caps, per module, You have to build one of these and set it up properly to appreciate how incredibly transparent they sound along with vast amounts of power
if needed and for hours at a time without any thermal problems.
Im not familiar with Aussie watts but I know what a sound pressure wave feels like
  Reply With Quote
Old 11th April 2008, 12:19 PM   #37
diyAudio Member
 
Join Date: Feb 2008
Quote:
Not all diodes drop the standard 0.7v. This value is only a convenience to make calculations easier.
The volt drop or Vf is dependent on the current flowing. Its not like a resistor; the Vf/If graph is not linear and Vf is not a constant for all values of If.
The 1N914 for instance has a Vf of 1v at 10mA forward current whereas the 1N400x series have a Vf of 0.7v at an If of 10mA and reaches 1v Vf at 1A If! Different types of diodes will have less or greater forward voltages. Similarly the transistor Base Emitter diode Vf is not a constant 0.7v for all types of transistors! This again depends upon the current flowing through the BE diode and can be lower than the assumed 0.7v. Actual values can be obtained from datasheets.
Thanks, but I'm into amplifier designing since many-many years, I already know these!
The current flowing through the bias diodes at 38V rails ~17mA. At this current an 1N914 drops ~0.78V. Then the total bias voltage is 1.5...1.6V. Still not enough for a darlington-EF OP stage (2.2...2.4V needed as I wrote it before)!

Quote:
The 1N914 for instance has a Vf of 1v at 10mA forward current
Check datasheet, at 10mA If an 1N914 drops ~0.72V. Not 1V as you wrote!
  Reply With Quote
Old 11th April 2008, 06:34 PM   #38
Bone is offline Bone  United Kingdom
diyAudio Member
 
Join Date: Jun 2004
Location: Dorset, UK
Quote:
Originally posted by Andy L. Francis



Check datasheet, at 10mA If an 1N914 drops ~0.72V. Not 1V as you wrote!
You were obviously looking at a different manufacturer's datasheet to me.




Tony
  Reply With Quote
Old 11th April 2008, 07:13 PM   #39
diyAudio Member
 
Join Date: Feb 2008
Ohh Bone, sorry then!
I observed Fairchild's papers.
  Reply With Quote
Old 12th April 2008, 06:54 AM   #40
Muff is offline Muff  Australia
diyAudio Member
 
Join Date: Mar 2008
Send a message via AIM to Muff Send a message via MSN to Muff Send a message via Yahoo to Muff
Quote:
Originally posted by Greg Erskine


Hi Glen,

I'm not sure if this PCB is close to the original, but on 25VDC rails I found the VAS transitors to be the one to worry about heat wise.

I was surprised, with a few mods, this simple amp sounded quite good. I preferred it to the Gainclones I was builting at the time. It definitely responded well to a little more bias and a bootstrap on the VAS.

BTW: The power traces are quite thin as you mentioned.

regards
This is the original ETI-1430 board from Graham Dicker.
The image is actual size.



Click the image to open in full size.
  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
Sub Amplifier Module Manufacturers schmeet Subwoofers 6 4th October 2011 03:23 PM
Got 2x65VAC PS, any idea on DIY amplifier module? barco Solid State 2 24th October 2008 05:31 AM
Help me troubleshooting amplifier module fosforo Solid State 15 7th November 2006 10:59 AM
Equilog PWM amplifier module? TroelsM Class D 7 11th November 2005 11:32 PM


New To Site? Need Help?

All times are GMT. The time now is 05:35 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