Quoll - a balanced, discrete IV stage - diyAudio
Go Back   Home > Forums > Source & Line > Digital Source

Digital Source Digital Players and Recorders: CD , SACD , Tape, Memory Card, etc.

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 October 2008, 12:49 PM   #1
diyAudio Member
 
PigletsDad's Avatar
 
Join Date: Jul 2007
Location: South Worcestershire
Default Quoll - a balanced, discrete IV stage

Attached below is a schematic for a discrete I/V stage, intended to be used with the Twisted Pear Buffalo or COD DACs. It uses Nelson Pass' supersymmetry concepts, with a couple of twists of my own. The gain block effectively has 5 terminals - ground sense, Input A and B, Output A and B. In use feedback loops are closed by putting resistors from Output A to Input B and vice versa.

The symmetry of the output is maintained by a separate path that compares the outpoint mid point with ground. In principle, if an application needed the complementary outputs to move centred on some other voltage, this could be fed into the base of Q1 instead.

The schematics have been checked by simulating the extracted Spice file, but not built.

The most important idea is in the next post, which will show how the basic block is used.

By the way, a quoll is an Australian marsupial carnivore, about the size of a cat. I saw them on holiday in Tasmania, and they most attractive. See
Wikipedia for more details.
Attached Files
File Type: pdf balancediv.pdf (10.9 KB, 360 views)
  Reply With Quote
Old 21st October 2008, 12:53 PM   #2
diyAudio Member
 
PigletsDad's Avatar
 
Join Date: Jul 2007
Location: South Worcestershire
The file attached here shows how the bock would be used.

Note that the dominant pole compensation components are placed before any active devices, at the "virtual earth" points. This removes high frequency rubbish before it encounters any devices that could possibly overload, fixing the major objection to feedback based IV circuits.

For a Buffalo, about 180 Ohms for RF and RF1 should give roughly Red Book levels.

Just a reminder, this has only been simulated, not built!
Attached Files
File Type: pdf useiv.pdf (4.0 KB, 204 views)
  Reply With Quote
Old 21st October 2008, 03:03 PM   #3
diyAudio Member
 
Russ White's Avatar
 
Join Date: Jan 2005
Location: Nashville, TN, USA
Send a message via Yahoo to Russ White
PD,

Clever.

That's very similar to the "original" counterpoint design I presented in the "Haiku" thread where I took feedback at the bases of the input pairs. I used a folded cascode where you are using a resistor load. I also used a diamond buffer type output stage.

I am sure the cct would work fine. Mine worked very well indeed, but in the end I decided I did not want to use Nelsons IP, and would try to create some of my own.

You will need a way to trim the output offset unless you can match things very very well.

Good work!

Cheers!
Russ
__________________
Less pulp more juice Twisted Pear Audio.
  Reply With Quote
Old 21st October 2008, 03:10 PM   #4
diyAudio Member
 
PigletsDad's Avatar
 
Join Date: Jul 2007
Location: South Worcestershire
Good point about the offset.

Because the DC gain is low (about 1) and the impedances very low, the offsets will be dominated by the Vbe mismatches of the differential pairs. If they come from the same batch, I think you would hope for less than 20mV, which should be acceptable for many applications.

If lower offsets than that are needed, the pairs of 47 ohm emitter resistors can be swapped for a trim pot.

I went for resistor loading, rather than folded cascode, as it gives some extra CMRR in the second stage.
  Reply With Quote
Old 21st October 2008, 03:23 PM   #5
diyAudio Member
 
Russ White's Avatar
 
Join Date: Jan 2005
Location: Nashville, TN, USA
Send a message via Yahoo to Russ White
Yes, like any opamp used for I/V, this one has a high open loop impedance, and low closed loop impedance.

I tried resistor loading too, but found it did not work as well for me.

Also you would likely need to augment the output devices to retain decent THD into lower impedance loads.

I also would look closely at how the circuit handles the fast slewing of the DAC as well as capacitive loads. This is the one major weakness of direct I/V using massive feedback.

How will the cct react to a short at the output?

Build one and give it a shot.

My discrete SUSY opamp I/V sounds excellent but is essentially the same as a THS4131.

Cheers!
Russ
__________________
Less pulp more juice Twisted Pear Audio.
  Reply With Quote
Old 21st October 2008, 03:45 PM   #6
diyAudio Member
 
PigletsDad's Avatar
 
Join Date: Jul 2007
Location: South Worcestershire
Quote:
Originally posted by Russ White
Yes, like any opamp used for I/V, this one has a high open loop impedance, and low closed loop impedance.

I tried resistor loading too, but found it did not work as well for me.

Also you would likely need to augment the output devices to retain decent THD into lower impedance loads.
Sure - I think Diamante was designed to drive headphones? I wanted to keep things simpler, and save at least one stage by targetting line level loads (although it drives say 600 Ohms very nicely). This is a deliberate trade-off - I didn't want to redo you work from Diamante!

Quote:
I also would look closely at how the circuit handles the fast slewing of the DAC as well as capacitive loads. This is the one major weakness of direct I/V using massive feedback.
I have studied this extensively, and this is why the compensation caps are in front of the input stages; you literally cannot make this slew rate limit with any signal in the DAC output range, no matter how fast; the dominant pole lies before the first gain stage.

The downside of doing this is that noise floor rises above the pole frequency, but I view this as a viable tradeoff at these signal levels.

Capacitive loads up to several nF are OK. Obviously a big enough load can make it unhappy at high frequencies - but it can drive tens of meters of cable without trouble.

Quote:
How will the cct react to a short at the output?
As shown, with loss of the magic smoke! In practice, I would put say 50 Ohm stoppers in the output lines to isolate it from cable capacitance, and these will just about save the output devices. This also gives more or less matched impedance drive to STP.
  Reply With Quote
Old 21st October 2008, 03:51 PM   #7
diyAudio Member
 
Russ White's Avatar
 
Join Date: Jan 2005
Location: Nashville, TN, USA
Send a message via Yahoo to Russ White
You will have to explain how you accomplish the pre-input filtering without raising the input impedance.

In the scheme you show in you second pic both the negative feedback and the input will be equally shunted to ground.

I just simulated, and this does not produce an effective filter.

I also have tried this with fully symmetrical opamps, it does not work.

To make it work, you need an input resistor after the filter cap. this will raise input impedance.

Also I was not referring to Diamante, to my opamp version of counterpoint.

I will try to dig up a link.

Cheers!
Russ
__________________
Less pulp more juice Twisted Pear Audio.
  Reply With Quote
Old 21st October 2008, 04:02 PM   #8
diyAudio Member
 
Russ White's Avatar
 
Join Date: Jan 2005
Location: Nashville, TN, USA
Send a message via Yahoo to Russ White
Default Discrete Opamp IV

http://www.diyaudio.com/forums/attac...amp=1204571940
__________________
Less pulp more juice Twisted Pear Audio.
  Reply With Quote
Old 21st October 2008, 04:13 PM   #9
diyAudio Member
 
PigletsDad's Avatar
 
Join Date: Jul 2007
Location: South Worcestershire
The caps are not for filtering - they are for stability!

If you want filtering, you add a cap across RF; I have simulated this a bit but need to do more.

For simplicity, lets start by considering a single ended case, with an inverting amplifier.

Consider the parallel combination of feedback resistor and input resistor (if any, say 200 Ohms for Sabre) as Thevenin equivalent source, with the FB loop open, but the far end of the feedback earthed (or otherwise driven from a low impedance). This interacts with the capacitor, to give HF rolloff. If we think of the input terminal of the circuit as lying before the equivalent source resistance of the Thevenin source, we find that open loop response of the amp rolls off, with a dominant pole set before the active device, giving overall stability without needing to put a roll-off at some interior point.

Closing the loop flattens the response (of course).

I have to go home now (wife stuck in traffic, dogs to walk, horses to muck out), but will put some simulations together tomorrow to show what I am getting at.
  Reply With Quote
Old 21st October 2008, 04:22 PM   #10
diyAudio Member
 
Russ White's Avatar
 
Join Date: Jan 2005
Location: Nashville, TN, USA
Send a message via Yahoo to Russ White
Quote:
Originally posted by PigletsDad
The caps are not for filtering - they are for stability!

I have to go home now (wife stuck in traffic, dogs to walk, horses to muck out), but will put some simulations together tomorrow to show what I am getting at.
I see now what you are saying.

I will give this a shot. I should work even for THS4131.
__________________
Less pulp more juice Twisted Pear Audio.
  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
FS: 6-channel volume--discrete, balanced (APOX) tiroth Swap Meet 8 8th March 2005 03:42 PM
Wanted->Discrete balanced to unbalanced CMR converter Brian Guralnick Solid State 20 21st September 2003 03:32 AM
Balanced DC servo for balanced stage ? CheffDeGaar Solid State 8 9th July 2003 02:39 PM
Zen Balanced Line Stage Balanced vs Unbalanced macka Pass Labs 28 11th December 2002 06:18 AM
Linkwitz Transform in discrete balanced circuit? Orgonom Pass Labs 0 15th April 2002 08:30 PM


New To Site? Need Help?

All times are GMT. The time now is 08:29 AM.


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