driving Pass A40 - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 5th March 2001, 07:39 PM   #1
diyAudio Member
 
Join Date: Dec 2000
Location: Saskatoon, Sk, Canada
I have a question about the Pass A40. Can I use a passive pre amp to drive it. At this point I really only have one source my Cd player. If so what value of POT/Attenuator do I use? (There seems to be alot of different values to choose from)In the article NP states the A40 have a nominal input impedance of 40K ohms. This is my first amp and any help would be great.
Ward
  Reply With Quote
Old 5th March 2001, 08:50 PM   #2
diyAudio Member
 
Join Date: Feb 2001
Location: Columbia, SC
There's not going to be a hard and fast value for your pot. I'd say to try a 47k log (aka audio taper) pot (pull one out of your junk box or buy a cheap one). For solid state stuff (i.e. the CD player and the amp) you don't generally run high values like, say 100k. For tubes, however, values like that are normal.
If you like, you can use a lower value, say, 20k (but still log taper). It won't hurt a thing.
Try it and see if your CD player has enough output to drive the amp to a: a loud enough volume to suit you, or b: into clipping, whichever comes first. If the volume is too low, you're only out the time (and possibly the money if you bought a pot) it took to put it together. Then you'll know you'll need a line stage, although it's not likely that you'll need much gain. 6 to 12 dB should be more than enough. If your CD player has sufficient output, then you can consider splurging on a fancy pot and all the other nice stuff.
A linear pot (the most common kind) won't hurt anything either, but you'll get all your volume in the first quarter-turn; no fine control. It's a cheap trick that stereo manufacturers use to impress the unwary--salesman turns the volume a little bit, shows customer the knob, customer thinks: Wow, if it's that loud with the knob at 9:00, just think how loud it will be wide open! Ain't so. Caveat emptor. You can use a linear pot, should you have one on hand, to determine if your CD player has enough output, but I wouldn't think you'd want to live with one.
Good luck.

Grey
  Reply With Quote
Old 5th March 2001, 10:42 PM   #3
diyAudio Member
 
mefinnis's Avatar
 
Join Date: Feb 2001
Location: Adelaide, Australia
Having used an A40 for several years with a passive PA, the short answer is YES!

However (there is always a catch!), it depends upon what you are running into the amp. Most modern CD players have outputs around the 2V mark and this will *easily* drive the A40. If you are using a RIAA section or other device, then it will depend upon the output from this.

IMHO, if you are listening to CDs, then the use of an active PA is totally irrational.

I use an ALPS 20KLog "high quality audio" pot. These cost a little (actually a LOT) more, however one of the really salient lessons from my youth was the noise from standard pots - don't use one of these except for testing purposes!

BEST option is to buy a good quality (ie. gold contact) multi-position rotary switch and build a stepped control using metal film resistors. It was in fact doing this which showed-up the gross noise we were hearing from standard pots!

My current project is a passive PA based on the volume control section from Pass' Aleph O PA. This uses an 8bit-ADC and a series of relays through metal film resistors - have a look at the service manual ;-)

cheers, mark
  Reply With Quote
Old 6th March 2001, 03:42 AM   #4
diyAudio Member
 
Join Date: Dec 2000
Location: Saskatoon, Sk, Canada
Default thanks

Thanks for the help
  Reply With Quote
Old 6th March 2001, 10:31 PM   #5
Eric is offline Eric  United States
diyAudio Member
 
Eric's Avatar
 
Join Date: Nov 2000
Location: Central PA, USA
Default Driving the a40 directly

I was figuring on driving my a40 (when its complete, that is) directly from my Adcom GCD700 cd player. It has a high output of ~2.8v and also has a remote controlled variable output on it. Have to check the impedance, but I'm sure the level is high enough to get suitable volume from the finished amp.
  Reply With Quote
Old 7th March 2001, 01:07 AM   #6
diyAudio Member
 
mefinnis's Avatar
 
Join Date: Feb 2001
Location: Adelaide, Australia
I just ran a simulation of the A40 through Circuitmaker and You will hit the rails at about 2.5V P-2-P.

Assuming the rating from the Adcom is RMS-AC, the unit has output peak-2-peak at 4V.

As stated previously, if you are using a CD player, you are *highly unlikely* to require a pre-amp .... the only thing it is useful for is adding 0.01% THD to your signal ;-)

cheers, mark
  Reply With Quote
Old 7th March 2001, 06:07 AM   #7
diyAudio Member
 
Join Date: Feb 2001
Location: Columbia, SC
doktor,
Duh! I'm slapping myself on the forehead. I just remembered why everyone doesn't use pots instead of preamps on high-output sources. The pot reacts with the capacitance and inductance of the interconnect going to the amp. Depending on the level of the volume control (hence the source resistance seen by the interconnect), you will get variations in frequency response. How much will depend on the reactance of the cable.
A line stage, setting aside questions of expense and gain, also buffers the volume control, usually by placing a cathode/emitter/source follower as the final stage. This gives you low output impedance from the preamp and minimizes interactions with cables.
In theory, I suppose you could make a case that it might oscillate. In practice, I've never heard of it happening, so I doubt that you will face any problems in the practical sense. It's just something that you might want to be aware of from a performance standpoint.
I seem to recall that Nelson has some discussion on this same issue in the Bride of Zen writeup. I think he decided to put the pot at the back, even with cable-dependent things going on.
Sorry to be so slow getting around to this, but I haven't had more than about four hours sleep the last several nights running, and my brain is in molasses mode...

Grey
  Reply With Quote
Old 7th March 2001, 07:22 AM   #8
diyAudio Member
 
mefinnis's Avatar
 
Join Date: Feb 2001
Location: Adelaide, Australia
Above comment is quite true. Potentiometer + C_cable will form a RC roll-off filter.

Before we condemn this, recall that most pre-amps also have high roll-off filters. So, how big a deal is this?

For fun, I just measured the C_cable of an "average" cable from my box = 0.128nF. Lets assume our pot is at 50%R and it is a 10K or 20k unit. Using Circuitmaker to run a simulation, driving this with 2V input (out should be 1V), we see at the following frequencies:

For a 20K pot:
1kHz = 999.26mV
10kHz = 998.34
20kHz = 995.59
100kHz = 929.55

For a 10K pot:
20kHz = 998.56
100kHz = 978.87 (considerably better!)

If we take a 10K input impaedance for our power amp, this equates to less than 1dB at 100kHz (someone please check this, I'm no EE and could easily have stuffed the math bit up!!)

So, what do we learn from this:

1. Choose a lower value for the pot, 10K is better than 20K, but you can likely go slightly lower than this if you are building with metal film Rs.

2. Keep lead length to a minimum. If we halve the cable length, then we have half the C_cable (0.064nF) and our worst case for 20k is not 929.55 but 979.28mV. If you are going 20cm then use 25cm, not the standard 1m! (This *IS* a DIY list, just make one)

3. Cable quality *does* matter. You can do much better than the 0.128nF per meter of the cable I measured. Good quality cable should actually list "pF/meter".

AND ..... I still recon this is less of a sin than would be committed by most pre-amps ;-)

cheers, mark

[Edited by mefinnis on 03-07-2001 at 01:25 AM]
  Reply With Quote
Old 7th March 2001, 04:09 PM   #9
diyAudio Member
 
Join Date: Mar 2001
Location: High Point, NC
Should be easy to increase amp gain by lowering R5 a bit. Normally it's 10k and changing it to 7.5k or 5k would likely be just fine, just an idea.
  Reply With Quote
Old 7th March 2001, 05:07 PM   #10
diyAudio Member
 
Join Date: Dec 2000
Location: Saskatoon, Sk, Canada
Default making attenuator

Now you guys have me thinking about making one. I found the following link that calculates the values.

http://margo.student.utwente.nl/klaas/audio_step.htm

now is 3.33... dB step size ok? I know it probally depends on listening preferences but is this a good starting point. Also note the highest/lowest level dB. Any ideas? I also read that a make before break switch should be used? All the info so far has been great thanks.
Ward

  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
DIY Low Pass and Mid/High Pass (Bass Blocking) Filters runslikealpaca Everything Else 1 19th June 2009 11:59 PM
Paging Mr. Pass, Mr. Pass pull my finger please. khundude Pass Labs 2 5th December 2008 05:11 AM
Pass A-40 safe driving ESLs rick57 Planars & Exotics 6 5th January 2007 11:07 AM
Driving Pass Amps direct without pre-amp? With a Wadia, experience anyone? Thurax Pass Labs 3 24th December 2004 04:12 PM
My opinion on Pass Labs and Mr. Pass (Nelson) himself b_online Pass Labs 11 21st May 2003 01:39 AM


New To Site? Need Help?

All times are GMT. The time now is 06:56 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