The Secret of Tube Amplifiers Revealed - and much more! - Page 30 - diyAudio
Go Back   Home > Forums > Commercial Sector > Vendor Forums > Vendor's Bazaar

Vendor's Bazaar Commercial Vendors large & small hawking their wares

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 12th March 2012, 04:41 PM   #291
ETM is offline ETM  Finland
diyAudio Member
 
Join Date: Oct 2009
Location: Kirkkonummi
The measured output impedance of the above transconductor is seen here. Up to about 4 kHz, it ranges from 1k to 10k, reaching a minimum 200 ohms at 20 kHz. Above 400 Hz, Zout is quite inversely proportional to frequency (capacitive) and is mostly due to C6. At low frequencies, the value becomes limited by the regulators' line regulation properties.
Click the image to open in full size.

The frequency behavior of the adjustable regulators is also very satisfactory for amplification purposes. With the circuit shown, the response is less than 0.3 dB down at 20 kHz. The low limit is determined by C1 and also C5.

As for stability issues, the circuit hasn't shown any signs of instability with any loads from zero ohms to highly inductive loads and voice coils and with varying input and supply levels.

A nice and economic feature with using this type of transconductor is yet that the amp can be used in a low power mode when only low or moderate listening levels are needed. By switching resistors R11 and R13 out, the regulator bias current and hence power consumption becomes halved while the circuit still operates normally but only with halved current capability and sensitivity. This also halves the background hiss though it is hardly discernible at normal distance even in the full mode.
__________________
www.current-drive.info
  Reply With Quote
Old 12th March 2012, 05:19 PM   #292
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
Thanks for the info. I think square-law class A is a good candidate for current-mode amplifiers:

Linear Audio | your tech audio resource
  Reply With Quote
Old 14th March 2012, 04:31 PM   #293
UnixMan is offline UnixMan  Europe
diyAudio Member
 
UnixMan's Avatar
 
Join Date: Apr 2005
Location: Perugia + L'Aquila, Italy
Send a message via ICQ to UnixMan
Quote:
Originally Posted by ETM View Post
It seems that adjustable voltage regulators are quite well suited for implementing current amps.
I'd say that Depletion MOSFETs are much better at that...
__________________
Quote:
"We should no more let numbers define audio quality than we would let chemical analysis be the arbiter of fine wines." N.P.
  Reply With Quote
Old 14th March 2012, 04:44 PM   #294
UnixMan is offline UnixMan  Europe
diyAudio Member
 
UnixMan's Avatar
 
Join Date: Apr 2005
Location: Perugia + L'Aquila, Italy
Send a message via ICQ to UnixMan
BTW: an interesting project of a very different kind of current mode amplifier: "www.audiofaidate.org - Argento Vivo".

(sorry, Italian language. Try with Google Translate). You need to (freely) register to the forum in order to view attached images/files.
__________________
Quote:
"We should no more let numbers define audio quality than we would let chemical analysis be the arbiter of fine wines." N.P.
  Reply With Quote
Old 12th April 2012, 04:01 PM   #295
ETM is offline ETM  Finland
diyAudio Member
 
Join Date: Oct 2009
Location: Kirkkonummi
Some amendment to the amplifier above:

Like conventional amps, current-output ones also usually need some muting of the on/off switching transients, and the topology presented is no exception. However, in current mode this can be done by shorting the speaker leads instead of breaking. Also, bulky and themselves noise making relays are not necessary, for the task can be accomplished with power mosfets.

Below is a muting circuit suitable for this. When the BJT is in off state, the mosfet gates are at the supply potential, keeping the fets conductive with low on-resistance. When the BJT is conducting enough, the gates (and sources alike) get very low potential which turns off the fets thus releasing the speaker terminals. R1 with C provides a delay that keeps the muting on during the power-up situation. When the supply is falling, C is discharged through the diode and the BJT turned off (and the fets on) without delay.

Click the image to open in full size.

The control circuit driving the gates can be common for all channels. When using other supply voltages than 30V, only R2 needs to be adjusted to keep the threshold level suitable at about 80% of the full voltage.
_______

In the amplifier schematic, capacitor C9 can actually be left out, for it is not really necessary to reference the non-inverting input to ground here. This even improves the overall PSRR somewhat.

Quote:
Originally Posted by UnixMan
I'd say that Depletion MOSFETs are much better at that...
I only haven't found practical examples on this.
__________________
www.current-drive.info
  Reply With Quote
Old 12th April 2012, 04:42 PM   #296
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
Blog Entries: 2
Most people use opto-isolators for MOSFET relays. Omron has some good solid state relays.

Your circuit doesn't appear to work. If the gates of the MFETs are driven, the parasitic capacitance will leak current into the sources and keep the gates and sources at the same potential. Where is the gate drive current loop? Furthermore, as the output voltage approaches the rails, the Vgs will be modulated, changing the relay resistance (but this may not matter if only used to short).
  Reply With Quote
Old 14th April 2012, 12:13 PM   #297
ETM is offline ETM  Finland
diyAudio Member
 
Join Date: Oct 2009
Location: Kirkkonummi
Hello keantoken

These issues do not pose problems, and the operation has been verified. In this use, there is no need to pay for opto-isolation. Wide availability of the parts is also a consideration.

When the gates are driven up, The parasitic Cgs capacitances do inject some microamperes to the source point, but due to the internal bulk diodes of the mosfets (not drawn) the source potential cannot rise much higher than the lower of the two drain (speaker) potentials. When the gates are driven down, the gate protection zener becomes forward-biased pulling the source point close to ground potential.

In the switch-off state, the parasitic capacitance loading the drains is less than a nanofarad, which can also be neglected. (The drain potentials must, of course, remain higher than the control circuit ground.)
__________________
www.current-drive.info
  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
$7500.00 chipamp revealed sendler Chip Amps 36 27th February 2009 01:25 PM
tube amplifiers for sale.... Nanook Vendor's Bazaar 0 30th March 2008 05:37 AM
why the fad with tube amplifiers? XxGoKoUxX Car Audio 20 27th February 2008 11:21 AM
Altec Tube Amplifiers peter hawthorne Tubes / Valves 0 16th August 2007 10:31 AM
DC Tube Amplifiers SY Tubes / Valves 14 1st November 2006 01:59 PM


New To Site? Need Help?

All times are GMT. The time now is 09:12 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