LM3886 GC with LM4562 tone control - Page 4 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 17th January 2013, 09:49 PM   #31
diyAudio Member
 
Join Date: Dec 2012
Location: Michigan
Quote:
Originally Posted by blu_glo View Post
are you doing switched inputs too?
I thought about having multiple inputs, but i think i'm just doing one for the time being. Maybe a future upgrade.
  Reply With Quote
Old 17th January 2013, 09:50 PM   #32
diyAudio Member
 
Join Date: Dec 2012
Location: Michigan
Quote:
Originally Posted by blu_glo View Post
buffer-tone-volume-power amp, in that order. make sure suitable dc blocking caps on input and output of volume control.
you would want to bypass buffer and tone and if you make volume control 22K then input impedance still stays the same
So caps on both sides of the pot? No caps in front of the buffer?
  Reply With Quote
Old 17th January 2013, 10:18 PM   #33
blu_glo is offline blu_glo  England
diyAudio Member
 
blu_glo's Avatar
 
Join Date: Aug 2005
Location: London, England
Quote:
Originally Posted by mxriddler View Post
So caps on both sides of the pot? No caps in front of the buffer?
yes caps on input and between all stages.
  Reply With Quote
Old 18th January 2013, 04:34 AM   #34
diyAudio Member
 
Join Date: Dec 2012
Location: Michigan
Ok, here's my first attempt at putting it all together. I'm sure I have errors, still working on it. Working on labeling too.

Am I on the right track?

Also, I've noticed that in some of the schematics I've been looking at there appear to be filters along the signal path. Like a resistor and capacitor in parallel to ground. Are these needed and where?
Attached Images
File Type: png LM3886 wTone V1.png (72.8 KB, 234 views)
  Reply With Quote
Old 18th January 2013, 05:34 AM   #35
blu_glo is offline blu_glo  England
diyAudio Member
 
blu_glo's Avatar
 
Join Date: Aug 2005
Location: London, England
Quote:
Originally Posted by mxriddler View Post
Ok, here's my first attempt at putting it all together. I'm sure I have errors, still working on it. Working on labeling too.

Am I on the right track?

Also, I've noticed that in some of the schematics I've been looking at there appear to be filters along the signal path. Like a resistor and capacitor in parallel to ground. Are these needed and where?
I would put a DC blocking capacitor between buffer and tone control stages. Ditto after the volume control.
Not putting a capacitor on the 680 ohm feedback ground of the power amp may cause a small amount of DC offset on the speaker output. This may or may not be significant.
Generally speaking a series (in the signal path) 1K resistor followed by ~ 220pF cap from signal to ground might be unsed on inputs from the outside world, to help prevent RF getting in.

The Preamp part as it stands has no gain. This means this design may work quieter at a given volume level than you'd normally expect. - It's not critcal - but a gain of approx. 4x will correct this. This is easily implemented by using a 82K resistor in the feedback loop of your buffer cirrcuit. (second resistor in from the left!!)
  Reply With Quote
Old 19th January 2013, 02:38 AM   #36
diyAudio Member
 
Join Date: Dec 2012
Location: Michigan
Quote:
Originally Posted by Mooly View Post
Yes you could do. I think you mean taking the feedback from the other end of the 10 ohm in the network.

Tbh it's an area of design I have never really looked into in great detail.
Wouldn't that likely lengthen the NFB loop quite a bit? I think the data sheet says to keep it as short as possible. Seems like I've seen many solder the resistor right to the pins of the chip to keep it as short as possible.
  Reply With Quote
Old 19th January 2013, 04:43 AM   #37
diyAudio Member
 
Join Date: Dec 2012
Location: Michigan
Quote:
Originally Posted by blu_glo View Post
I would put a DC blocking capacitor between buffer and tone control stages. Ditto after the volume control.
Not putting a capacitor on the 680 ohm feedback ground of the power amp may cause a small amount of DC offset on the speaker output. This may or may not be significant.
Generally speaking a series (in the signal path) 1K resistor followed by ~ 220pF cap from signal to ground might be unsed on inputs from the outside world, to help prevent RF getting in.

The Preamp part as it stands has no gain. This means this design may work quieter at a given volume level than you'd normally expect. - It's not critcal - but a gain of approx. 4x will correct this. This is easily implemented by using a 82K resistor in the feedback loop of your buffer cirrcuit. (second resistor in from the left!!)
Updated plans attached.

Fortunately, I found the Overture_Design_Guide15.xls file online that really helped out in picking values for the amp section. Correct me if I'm wrong, but the values I have now should work well with my line level source, gain being at 21 and 1.2v input before distortion.

You mentioned adding 4x gain at the buffer, is that still needed? Wouldn't that then make the volume change when switching tone on and off? Or was that suggestion to compensate for some volume loss in the tone section?

Have you found an RF filter on the signal input is usually necessary?

I think the last thing is how to calculate the values for C1, C2, and C3. What formula should be used?

Thanks!
Attached Images
File Type: png LM3886 wTone V2.png (95.1 KB, 213 views)
File Type: png Screen shot 2013-01-18 at 11.20.32 PM.png (191.3 KB, 203 views)
  Reply With Quote
Old 19th January 2013, 07:38 AM   #38
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by mxriddler View Post
Wouldn't that likely lengthen the NFB loop quite a bit? I think the data sheet says to keep it as short as possible. Seems like I've seen many solder the resistor right to the pins of the chip to keep it as short as possible.
The "output end" of the chain (the 10 ohm or 2.7 ohm in your new diagram) is at low impedance and unaffected by whats going on around it and any extra PCB length. It is the inputs of the chip that would be sensitive to extra length and the risk that brings of stray pickup. I wouldn't like to say yay or nay to the technique without verifying it and designing a suitable PCB to match. Stick to what you have drawn.

The times 4 buffer would bring a huge gain change as you say.

C1 and C2 (C3 ???? in the tone control) The inverting buffer (in the tone stage) has an input impedance determined by the 20K. So that determines how big C1 has to be and that in turn is determined by you and what you deem a suitable cut off point. Typically a designer would work to <1Hz for these. So that implies at least 10uf. Why not make the 20k's bigger ? Say 180K. That brings the added bonus of a higher input impedance too and allows a 1uf to be used. C2 needs to be bigger as it drives the low impedance tone network. I would use a 100uf electrolytic here. There is no right and wrong in all this. They are just choices. The 1k and 33uf in the power amp come in at around 5Hz (which is fine for the final power amp stage)

Just spotted an error... The 2 uf cap in the power amp. You need a 20K (or 19 to be correct) to ground from this cap. The pin is floating at DC otherwise and the amp can not establish its DC conditions.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 19th January 2013, 07:58 AM   #39
blu_glo is offline blu_glo  England
diyAudio Member
 
blu_glo's Avatar
 
Join Date: Aug 2005
Location: London, England
I recommended 20 K resistors on the grounds of keeping noise low. to this end and if at all possible I therefore tend to stick to below 100 K . the power amp inputs did loose their dc path to ground when I believe on my suggestion the feedback capacitor was added.
  Reply With Quote
Old 19th January 2013, 11:43 AM   #40
blu_glo is offline blu_glo  England
diyAudio Member
 
blu_glo's Avatar
 
Join Date: Aug 2005
Location: London, England
Quote:
Originally Posted by mxriddler View Post
Updated plans attached.

Fortunately, I found the Overture_Design_Guide15.xls file online that really helped out in picking values for the amp section. Correct me if I'm wrong, but the values I have now should work well with my line level source, gain being at 21 and 1.2v input before distortion.

You mentioned adding 4x gain at the buffer, is that still needed? Wouldn't that then make the volume change when switching tone on and off? Or was that suggestion to compensate for some volume loss in the tone section?

Have you found an RF filter on the signal input is usually necessary?

I think the last thing is how to calculate the values for C1, C2, and C3. What formula should be used?



Thanks!

Hi the x4 preamp gain is normal accross the field if you are connecting other devices, it's best to have some gain available. However if your particular application is one -input/use then you can try your original values and see if they're OK.
Yes you're right - increasing the gain of the buffer wasn't the best option, the level would change when switched in/out, well done for spotting that!
There should be no loss in the tone control section with controls set to "flat", i.e.mid-position. in this case gain = x1 at all frequencies.
The passive kind of tone control normally has an insertion loss of x0.1 in mid postion but you have avoided this by opting for active. When including tone control circuits, active is the best in tracking and predictability with the opamp working its magic in the middle!!

Have I "found an RF filter to be useful" - well, I believe I have avoided picking up radio on external leads anyhow!

This subject has already been touched apon by Mooley, thanks, although I may repeat and/or exand on it we seem to agree!
the formula you want is if we assume C (in Farads) R (Ohms) and frequency of interest F (in Hz or cycles a second):

C= 1 / (6.284 x R x F) [ 6.284 = 2 x pi (approx) ]

If, as Mooley suggests, you want to include subsonics you would probably use 1Hz as F - it is indeed what I tend to do!

If you use seperate sub or small speakers with no chance of producing subsonics, you can use even higher eg 5 or 20Hz

My suggested values would be C1 10uF; C2 100uF, Ci (power amp feedback) 220 uF all at least 35V non-polarised electrolytics if you can. It's a personal thing but i'm not a fan of using polarised electrolytics in signal paths with no DC (as all these positions are). You can however "get away with" ignoring me on that one!

you might want to add a 100K resistor from [junction C6/SW1] to ground 0V to minimise speaker thumps when activating the switch, also the "bypass line" should go to the other side of C1.
Mooley also pointed out quite correctly that putting the feedback capacitor in lost your power amplifiers DC reference (can be on either of the two input pins but must be on one or the other). This is remedied by a >>22K say 100K resistor value (somewhat arbitrary but must be much greater than the volume resistor in value).

Last edited by blu_glo; 19th January 2013 at 11:51 AM.
  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 control argonrepublic Solid State 11 13th November 2011 02:32 AM
To tone control or not to... dagerman Tubes / Valves 12 27th June 2011 07:15 AM
Tone control argonrepublic Solid State 0 28th February 2010 01:12 AM
i need preamp schematics w/o tone control or volume control hernanstafe Parts 7 30th January 2008 04:12 AM
Tone Control for my GC aditaquil Chip Amps 5 7th January 2005 12:39 AM


New To Site? Need Help?

All times are GMT. The time now is 11:21 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