Power amp update & preamp questions..

Ok, got some work done on my 2x20w '95 JLH amp... right now i'm saving up for a custom transformer and aluminium to build the chassis (which doubles as the heatsink) arround it. What's done are all the electronics except for the power supply: the twin amp boards, a remote power on circuit (using an external signal from the preamp, a small transformer and a power relay), dual power meters using the venerable LM3915 and an relay output protection based on an article at the ESP site (which triggers on DC through the speakers and power-on & power-off thumps). Lots of lights and switches... can't wait to get this thing togheter! :D

Now, i started working on the preamp... i wanted the power and input switching to be digitally controlled via relays, and wondered, why not driving the volume digitally as well? I've seen two aproaches to this:

- Run the signal through a multiplexer acting as a variable voltage divider. Dirt cheap, but of course, i have my doubts about signal degradation
- Use a series of relays and resistors on ladder configuration; no signal degradation, but i'm already spending a lot on relays...
- Getting a triple-gang pot, and using a A/D converter on the third pot to drive the front volume display (i know, i know, but it *should* look neat atleast :)

So, any experiences with any of the above? I don't want to complicate my life any more already, but since i have the digital board already, a few more ics wouldn't kill me.
 
I got a update on new products from Texas instruments by Email today discribing the new Burr Brown digital volume control. The link does not seem to be working, but you may be interested in looking at this device by searching thier web site.

Here is a partial copy of the TI news letter.

Texas Instruments Unveils Industry's Highest Performance Audio Volume Control IC for Professional Audio Systems -- PGA2310
TUCSON, Ariz. (Oct. 19, 2001) -- Designed for a wide
variety of professional recording and playback
applications, Texas Instruments (TI) (NYSE: TXN)
introduced the industry's highest performance audio volume
control IC (Integrated Circuit) from the company's Burr-
Brown products division. Featuring 120dB dynamic range,
0.0004 percent distortion (THD+N), -126dB interchannel
crosstalk, and +/-15V analog power supplies, the PGA2310
improves system performance in professional audio
applications such as digital mixing consoles, multi-track
recorders, broadcast studio equipment, high-end A/V
receivers, effects processors, musical instruments, and
high-end car audio systems. See
http://www.ti.com/sc/aap6908u.

John Fassotte
Alaskan Audio
http://www.audioamps.com
 

paulb

Member
2001-06-01 4:53 pm
Calgary
John,
This device probably deserves a new thread. There have been several discussions at this site about digital control of volume. This one looks very good. I've had some bad experience with Analog Devices DC-operated volume controls, but am still interested in something like this.
Looks pretty pricey ($19 CDN from Digi-Key, in stock already!), plus you need a micro to run it. Nevertheless, it may interest many DIYers. Have you (or any others out there) got any plans to try it out?
 
Digital volume control

paulb,

In reference to my previous post for the new TI/Burr Brown PGA3210 digital volume control IC.

I have some interest in testing this device in the future. However I have no time to devote to such a project at the present time since I’ am working hard on a all discrete version of my MOSFET class a power amp.

Since I have very little time to attack new projects right now my contribution would be limited to helping others to develop a test bed for this device when my time allows.

I HAVE STARTED A NEW THREAD ON THE PGA3210 Digital volume control. Please post all replies in regards to the PGA3210 there. Thanks.

[Edited by alaskanaudio on 10-21-2001 at 07:18 PM]
 
Pete Fleming said:
An LM1972 is an alternative and is used in the Arcam Alpha 10 amplifier.


Hrm, this could do the trick... according to National Semiconductor's site (http://www.national.com/pf/LM/LM1972.html):

- Total Harmonic Distortion + Noise: 0.003% (max)
- Frequency response: 100 kHz (-3dB) (min)
- Attenuation range (excluding mute): 78dB (typ)
- Differential attenuation: ±0.25dB (max)
- Signal-to-noise ratio (ref. 4 Vrms): 110dB (min)
- Channel separation: 100dB (min)

I'll see if i get one of these IC's this week and try it to see if it sounds any good. Thanks!
 
The Arcam Alpha 10 has received good reviews for an amp in its price range. I notice that bypassing the pre-amp section of the 10 does reduce the noise floor, however the noise may not be coming from the LM1972. I suspect it's actually mainly from the MAX4051 input switches but I've never bothered putting an oscilloscope on to see. The noise is not noticeable on speakers, but is when using headphones.

Cheers,

Pete
 
Pete Fleming said:
The Arcam Alpha 10 has received good reviews for an amp in its price range. I notice that bypassing the pre-amp section of the 10 does reduce the noise floor, however the noise may not be coming from the LM1972. I suspect it's actually mainly from the MAX4051 input switches but I've never bothered putting an oscilloscope on to see. The noise is not noticeable on speakers, but is when using headphones.

Yeah... well, this is why i wanted to stay away from "volume pot" IC's at first, but i have to draw a line somewhere. My input switching is controlled by relays and a rather simple digital circuit, but i want to stop using them there. I've seen some other approaches i'll be trying this week to see how they turn out (one that looks promising was mentioned on the PGA3210 thread, check http://members.tripod.de/MaikHerzog/audio_projects/preamplifier/preamp_text.html).
 

mlloyd1

Member
Paid Member
2001-02-25 7:10 pm
Northern Iliinois
It possible to use other MDACs as volume controls besides the serially controlled CS3310 and LM1972. Analog Devices (audio specific - AD7111/A, AD7112/A, etc.), Maxim (MAX502?, etc.) and others make MDACs that are parallel input, so you don't have to use a microcontroller unless you want to. "Static" logic should have less digital hash floating around than a clocked microcontroller. The other option is to have the micro "go to sleep" a short time after a function is executed. I think Mark Levinson actually does this in their current preamp line.

Using the micro certainly does help if you want to have some fancy displays, though!

Pete: By the way, is there a schematic of the Arcam out there somewhere or did you just "pop the top" and mention what you saw? It's nice to see someone actually using some new and improved analog switches from Maxim. I wonder if any folks with Hafler 915 preamps ever do any mods like changing to a better analog switch.


Michael
 
mlloyd1 said:
It possible to use other MDACs as volume controls besides the serially controlled CS3310 and LM1972. Analog Devices (audio specific - AD7111/A, AD7112/A, etc.), Maxim (MAX502?, etc.) and others make MDACs that are parallel input, so you don't have to use a microcontroller unless you want to. "Static" logic should have less digital hash floating around than a clocked microcontroller. The other option is to have the micro "go to sleep" a short time after a function is executed. I think Mark Levinson actually does this in their current preamp line.

Using the micro certainly does help if you want to have some fancy displays, though!

I'll give a shoot at the shunt resistors driven by transistors to see how it works first. Looks promising, and if it works ok, i'll be easy to implement. BTW, my "fancy display" right now is a large (LARGE) green LCD... gotta love those lights :D


Pete: By the way, is there a schematic of the Arcam out there somewhere or did you just "pop the top" and mention what you saw? It's nice to see someone actually using some new and improved analog switches from Maxim. I wonder if any folks with Hafler 915 preamps ever do any mods like changing to a better analog switch.

Yep, i'd love to see it myself too!
 
Michael, if you contact Arcam customer service they may be able to help you with a service manual. I'm not saying they will, but they really are a nice bunch of people whom I admire for being able to bring quite good designs to the masses. Just the same they're a commercial operation so they may not feel it's in their interests to spew their service manuals over the internet. Personally I'm with Nelson Pass, helping people in this area is some of the best advertising you can get (ever listened to an audio enthusiast when asked by a layman which hi-fi system they should buy!), however not all companies share this attitude. Good luck.

Cheers,

Pete