A preamp with tone controls - 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 11th February 2004, 05:11 AM   #1
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Default A preamp with tone controls

I am trying to build a preamp with tone controls, based on the circuits in Randy Slone's "Audiophile Projects Sourcebook." I've combined the circuit for his basic gain stage + loudness + volume control, with his circuit for a three-band tone control, and inserted a switch for bypassing the tone controls.

Even though the essential design is in most respects a straight copy from his book, I have many questions about the actual implementation. And those are the questions I'll list below, but first I thought I'd include the schematic and give some explanatory notes.

Tarun
Attached Images
File Type: png preamp-sch.png (15.4 KB, 1603 views)
  Reply With Quote
Old 11th February 2004, 05:31 AM   #2
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Default Some explanations about circuit

A lot of the explanations may be unnecessary if you have read Randy Slone's book from which I've got the schematic. Those of you who are beginners to audio amps like me, and who want to build a good preamp, may like his book. I know I learned a great deal from it.

Now for the notes:
  • The preamp uses two opamps per channel, one for the gain stage, and one for the tone controls. I've used one dual-opamp chip to implement the gain stages for both channels, and the second dual-opamp chip for both tone controls. (I know the crosstalk will probably be a bit higher this way, but I wanted easier PCB layout. I'm a newbie, and my free Eagle software can do only very small PCBs.) The diagram in the earlier post is only for the left channel.
  • The switch LSW-L is "Loudness Switch - Left." There is an identical "LSW-R" on the right channel.
  • The "VOL-L" pot is the volume pot for the left channel. I am intending to use a linear volume pot, followed by a smaller-value balance pot, to get approximately log taper, as per the idea given by Rod Elliott on his Website. His approach to connecting a balance pot is to use a mono pot with the wiper connected to ground, not a contra-rotating dual-ganged stereo pot.
  • IC1B is a non-inverting gain stage. Its output goes out via R8 (100 Ohms) to the output terminal (OUT-L). It also feeds the non-inv input of the tone controls.
  • TREBL-L, MID-L, and BASS-L are three tone control pots, each 100K, in the feedback loop of the second opamp, labelled IC2B. I hope I have not made any mistakes in copying the schematic of this stage. The output goes out at TONEOUT-L.
  • There are a lot of solder-pads, e.g. VSL1, VSL2, VSL3, LSL1, LSL2, LSL3, etc. All these are meant to solder the shield of shielded hookup wire which I'll use to connect the various switches and pots; all these pads connect to ground.
  • The input selector stage is not shown in the schematic, because it'll be implemented using a rotary switch and point-to-point wiring, and the selected input will feed into the schematic at INPUT-L. Similarly, there's a final switch at the output, which will select between TONEOUT-L/R and OUT-L/R. That switch too is not shown here.
  • I have put one pair of caps for supply decoupling when the supply rails enter the PCB, and two pairs of caps for each opamp chip.
Hope all this makes sense. Comments please?

Tarun
  Reply With Quote
Old 11th February 2004, 05:40 AM   #3
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Default The PCB for the preamp

The PCB layout that I've done so far barely fits everything onto one PCB for both channels. (Remember, I use Eagle's free version, where PCBs cannot exceed 3.2"x4".) It's a single-sided PCB; all red tracks will be executed using jumpers.
  • It's a single-sided PCB. The top half handles the entire work for the left channel, and the bottom half handles the right.
  • It expects clean regulated 15-0-15 (or thereabouts) supply rails. The PSU circuit is not on this PCB.
  • There is no separation between power supply ground and signal ground... there's no star grounding. Can't see how I can do it without increasing board size or adding half a dozen or more jumpers.
  • All switches and pots are assumed to be panel-mounted, i.e. off-board. Hence, shielded hookup wires are expected to be used for all that wiring. The shields of such hookup wires will need to be grounded. Solder pads have been provided for the worst-case scenario for such wires, i.e. assuming that all your hookup wires are just 1 core+ shield. In reality, it is likely that you'll get 2-cores + shield or 3-cores + shield, in which case you won't need to use all those shield solder-pads. They can be left unused.
How does this look? Will it work? Comments invited. This is my first preamp PCB, so please don't lose patience with me.

Tarun
Attached Images
File Type: png preamp-pcb.png (36.0 KB, 1450 views)
  Reply With Quote
Old 11th February 2004, 05:53 AM   #4
diyAudio Member
 
Bill Fitzpatrick's Avatar
 
Join Date: Jun 2001
Location: Eugene, OR
Personally, I would make every effort to come up with a layout that would allow for separate signal and power grounds.
  Reply With Quote
Old 11th February 2004, 06:00 AM   #5
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by Bill Fitzpatrick
Personally, I would make every effort to come up with a layout that would allow for separate signal and power grounds.
Ouch. I was dreading someone would say this. But I guess I'd better struggle now than get all sorts of problems when I build it.

Tell me, do the shield grounds for the shielded hookup cables qualify as "power" or "signal" ground? I really can't figure this out.

Thanks for the feedback. Anything else you think I should fix?

Tarun
  Reply With Quote
Old 11th February 2004, 06:13 AM   #6
diyAudio Member
 
Bill Fitzpatrick's Avatar
 
Join Date: Jun 2001
Location: Eugene, OR
The shield grounds are signal grounds.
  Reply With Quote
Old 11th February 2004, 07:15 AM   #7
ashwin is offline ashwin  India
diyAudio Member
 
Join Date: Apr 2003
Location: Bangalore
Hi Tarun,
it looks like a small section of the GND trace is going in a complete circle (see the top left corner of the PCB, basically encompassing C1, R3 and Vol1). I''m not sure if this is a good idea, the loop may pick up noise.

Also, I'm a bit surprised that you didn't simply mirror the pattern for one channel to get two. That's what I did for my phono preamp with no problems. Any reason why you didn't do this?

Also, you might want to put slightly bigger values in parallel with C17/C18 for the power supply entry point, maybe 10uF/100uF ? (not sure if this is really required, it's just something I do for my PCBs).

- Ashwin
  Reply With Quote
Old 11th February 2004, 07:59 AM   #8
diyAudio Member
 
runebivrin's Avatar
 
Join Date: Jul 2003
Location: Huddinge, Sweden
Send a message via MSN to runebivrin
Near as I can tell, this circuit has no power ground beyond what goes to the filter caps. There's no low impedance return from a loudspeaker to consider, so currents in the ground should be insignificant.

Rune
__________________
Do wizards use spell checkers?
  Reply With Quote
Old 11th February 2004, 08:13 AM   #9
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by ashwin
it looks like a small section of the GND trace is going in a complete circle (see the top left corner of the PCB, basically encompassing C1, R3 and Vol1). I''m not sure if this is a good idea...
You're damn right! I fixed it, attached below.

Quote:
Also, I'm a bit surprised that you didn't simply mirror the pattern for one channel to get two. That's what I did for my phono preamp with no problems.
I didn't even know such a thing was possible. It was only during this PCB design that I fully understood how to mark groups and move/cut/paste them. For the schematic layout, I did one channel and then did a group cut/paste to get the other channel, and thought myself totally brilliant. Then I used gateswap to get the unnecessary IC opamps out of the schematic and replace them with spare opamps of the earlier ICs, and considered myself positively Einsteinian. Now you tell me about mirroring of PCB? Sigh...

Quote:
Also, you might want to put slightly bigger values in parallel with C17/C18 for the power supply entry point, maybe 10uF/100uF?
Will do. Can't harm anything. As long as I can find the board space.

Thanks.
Tarun

PS: BTW, if anyone wants these layouts, just ask. I can send you the Eagle directory contents, zipped. Or I can send you PostScript or PDF of track side, component side legends, or solder mask, 1:1 or 1:2. Remember, it's work in progress, so you'll get whatever the current snapshot is. I won't post the files to the forum unless a lot of people ask; send me email.
Attached Images
File Type: png preamp-pcb.png (35.9 KB, 1286 views)
  Reply With Quote
Old 11th February 2004, 08:18 AM   #10
tcpip is offline tcpip  India
diyAudio Member
 
Join Date: Jan 2003
Location: Mumbai, India
Quote:
Originally posted by runebivrin
Near as I can tell, this circuit has no power ground beyond what goes to the filter caps. There's no low impedance return from a loudspeaker to consider, so currents in the ground should be insignificant.
That's exactly the point. I get conflicting views about the need for star grounding in circuits like this. I asked Randy Slone, one of the Gurus I have learned many things from, and he says it's not necessary for signal-level circuits. I asked Angshu, another of my Gurus, and he says that he always does star grounding, even for low-current stuff. Others on this thread seem to have one or other opinion. I don't know what to follow.

Tarun
  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
Tone Controls? fuzzjunkie Tubes / Valves 2 27th February 2008 07:49 PM
Dynaco PAS Preamp Tone Controls Justin_122 Tubes / Valves 0 7th October 2007 04:32 AM
Tone controls phn Parts 7 11th June 2006 03:03 PM
Tone controls mik Solid State 2 14th January 2005 03:46 PM


New To Site? Need Help?

All times are GMT. The time now is 07:20 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