Welcome, Guest. Please login or register.

December 03, 2020, 02:21:16 AM

Login with username, password and session length

Recent Posts

 

Author Topic: Help comparing preamp schematics - can you tell me why these sound different?  (Read 19607 times)

kvandekrol

  • Regular
  • **
  • Posts: 36
  • Chip Points: 1
    • View Profile
OK... as a preface, this isn't actually a guitar amp, it's an active preamp inside of a guitar. But the principles are all the same! I need help trying to figure out why two preamps with the same specs sound differently. I have attached schematics for both of them.

One is from a Fender Elite Stratocaster from the early 80's, and the other is the Eric Clapton mid-boost from the late 80's continuing to today. The Elite came first and then the Clapton boost was based on it. The two guitars sound very different, and though this could be caused by the body wood or the pickups, I have a sneaking suspicion that the preamps are actually voiced a bit differently.

I am not well-versed in schematics, so this is like requesting a translation, but I was mainly wondering if someone could compare these two schematics and tell me if there's anything obvious that might cause the two boards to sound different. It seems like different frequencies are being boosted, and it may just be a matter of switching out a couple of components for different values to get the change in sound. (The full story is that I have the newer Clapton preamp and I want it to sound more like the Elite, and was wondering if I could do it by swapping a couple of twelve-cent components.)

A couple of notes on the obvious differences I found:

 - The Elite diagram has a whole section for the dummy coil which cancels out hum in all positions, as well as a buffer to drive it. The Clapton model does not use a dummy coil as it's fitted with noiseless pickups, so this difference can probably be ignored.

 - The Elite uses a 50k linear taper pot for the mid-boost tone control, whereas the Clapton uses a 250k audio. What effect would this have on the sound? May account for some or all of the difference.

 - The Clapton ends up with a 25dB boost while the Elite only has 12dB. From what I've read this is just a difference of a couple of resistors. The total output is not an issue, the idea is just to compare the sound at the same boosted dB level.

I have a number of other documents I can upload if it would help - a parts layout for both boards and a detailed closeup of the Elite circuit board, front and back.

Thanks for any help you can provide! As I said, I'm fairly inexperienced in figuring this stuff out for myself, but I am very curious to see whether the boards are voiced differently.
« Last Edit: December 16, 2009, 08:56:22 AM by kvandekrol »

kvandekrol

  • Regular
  • **
  • Posts: 36
  • Chip Points: 1
    • View Profile
Attachments are being weird... here's the Clapton schematic, Elite to follow.

kvandekrol

  • Regular
  • **
  • Posts: 36
  • Chip Points: 1
    • View Profile
Elite schematic:

http://www.eledar.net/Music/walnelite/photos/elite%20pre%20schem2.jpg

(sorry, won't let me attach it for some reason)

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Hi kvanderkrol.
Post or link what you have: layouts, pictures, etc.
Both preamps *are* the same, just 2 versions of the same beast.
Quite an old design by the way, it can be done today with an TL072 or even a TL062 for longer battery life, but well, you don't fix what still works.
Besides that, if  ol' chap Eric uses it, that's all that matters, or so it seems.
Magic runs the world, or at least marketing.
I already have a basic idea of what to do, but will wait for your pictures/layout to be more certain.
It worries me a little that there is no bypass switch to send PUps straight to the output incase of battery death or some other electronic misbehavior.
Well, if Eric can live with that .....  8|

kvandekrol

  • Regular
  • **
  • Posts: 36
  • Chip Points: 1
    • View Profile
Attached the following:

clapton_layout.gif - parts layout of the Clapton preamp

Elite_PCboard_schematic.jpg - parts layout of the Elite preamp

elite_pcb_closeup.jpg - a very detailed photo of the Elite preamp, front and back

elite_wiring_diagram.jpg - the Elite's guitar wiring diagram showing which wires from the PC board go where - this may not be useful

ec_wiring_diagram.jpg - Clapton wiring diagram - again, may not be useful

Thanks for your willingness to help!

« Last Edit: December 16, 2009, 08:44:16 AM by kvandekrol »

phatt

  • Legendary
  • ******
  • Posts: 2148
  • Chip Points: 262
    • View Profile
Hi kvanderkrol,
               Ouch yes a tricky one to sort.
My guess, though they look similar they are very different.
Short answer is change the 250k pot back to 50k pot and it will be less boost,,,
As to how much?  Well you would just have to try it.

I don't believe that the *Dummy load* is just for noise cancelling, I think it was there for loading? Not sure?

The pup's are also very different as the *Elite* uses the tried and tested old pup design.

The Elite also has a very unique pup selector, being 3 push buttons instead of the 5 way.
If I remember correctly All 3 pups can be engaged at one time, any combination with Elite.

After a brief look at the two schematics there are at least 6 or more different values that would impart a different outcome to the tonal effect. C1, C2, C5, R4, R5, R13.
Edit (numbers from the clapton circuit)

IMHO far to much muckin about for very little benifit.
I have a friend that owned an Elite and the only great benifit was the onboard volume worked like a built in OD stomp box. kinda handy but yep an absolute pain when the battery went dead.
I agree with JMF comments, without a passive bypass mode it's un-nerving when the battery is low and you have to break into a big leed part.

Bare in mind that with these PCB's once you start unsoldering and resoldering the tracks can delaminate and you can end up with a dud pcb.
Just thought I'd mention it as some people forget that a lot of these old pcb's would be darn near impossible to replace now.
Phil.
« Last Edit: December 16, 2009, 10:11:01 AM by phatt »

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Hi kvanderkrol.
From what I see, although electronics can change sound somewhat, the *main* difference must lay on the very different PUps used.
Any way, the Clapton sounds *good*.
The two *big* differences are, like Phatt posted, the 50K Lin vs 250K Log/Audio pot and R13 Clapton (15K) vs. R13 Elite (47K) which accounts for the extra gain (Lower resistor= higher gain).
Anyway, I still suspect the main difference lies in the Pickups.
Laces are good but I still like classic single coils better.

kvandekrol

  • Regular
  • **
  • Posts: 36
  • Chip Points: 1
    • View Profile
phatt, would any of those components you listed affect the actual mid frequency that is boosted? To my ears the Elite has a bit more "nasal" tone to it... as you and JMF said, it's probably in the pickups, but I just wanted to make sure.

kvandekrol

  • Regular
  • **
  • Posts: 36
  • Chip Points: 1
    • View Profile
Also, I should mention that my guitar is a Powerhouse Strat, which is essentially a MIM Clapton. It uses the exact same preamp board except that its output has been reduced to 12dB like the Elite. (I couldn't find any schematics specific to the Powerhouse which is why I referenced the Clapton.)

It has American Standard pickups in it rather than Clapton's Lace Sensors. It also has the hum-canceling dummy coil just like the Elite, except it's wired in parallel right after the pickup selector rather than having a separate wire coming out of the preamp board like the Elite (thus no need for a buffer stage to drive it)

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Well, you have a *nice* guitar.
I guess that that dummy pickup, just wired in parallel with any one you select, completely unbuffered :o, must change the sound more than anything else.
Is it visible or just hidden under the surface?
Post some picture.

phatt

  • Legendary
  • ******
  • Posts: 2148
  • Chip Points: 262
    • View Profile
Also, I should mention that my guitar is a Powerhouse Strat, which is essentially a MIM Clapton. It uses the exact same preamp board except that its output has been reduced to 12dB like the Elite. (I couldn't find any schematics specific to the Powerhouse which is why I referenced the Clapton.)

It has American Standard pickups in it rather than Clapton's Lace Sensors. It also has the hum-canceling dummy coil just like the Elite, except it's wired in parallel right after the pickup selector rather than having a separate wire coming out of the preamp board like the Elite (thus no need for a buffer stage to drive it)

Hello again kvandekrol,
Found the time so have a look.
Response curves using *Your EC circuit* except I've simmed it with a 50k pot instead of the
250k pot.
First thing I noticed is that it's got heaps of cut/boost BUT way to high in the bandwidth to
be of any real use.  I would hardly consider this as a *Midboost* circuit,,Treble boost more
like it.

The 10Db of boost is only happening at 10kHZ which is at the very outside edge of what your guitar can possibly produce so any real mid boost your ears hear is only a couple
of Db at 3/ 4kHz. Heck even an hobby nob like me could do a better mid boost circuit. :D

As you may have noticed I don't get excited about active circuitry in guitars as a well
designed Amp can do a far better job of mid boost and they don't chew batteries  8|
but I'd guess from reading your post you really want to fiddle with this so having spent an
afternoon simulating this the best suggestion is Fiddle with the value of *C3 .0068uF* which is 6n8 or 6,800pF.

Try 10/ 15nF ,, even 18nF,, past that you may as well forget it. (but try more if you wish).
15n will make the boost more active from just under 1kHz which is still very high for my idea
of midrange but you will definitly notice a difference.

Just a note in case you have trouble with freq.
Midrange for guitar signals is 300/400 Hz. 700Hz is upper mid, past 1kHz you are in the treble range.

Be very aware that you can never get the *natural sound of the pups* with a circuit such as this because the treble boost (sorry mid) is created in Q1 and the mid pot just cuts or boosts this basic tone shape. It actually boosts all frequencies but just more emphisis on the high. (There is no *Flat point* where the frequency is left untouched by the active circuit. it's always on and altering the tone shape.)

Oh yes from my understanding the MIM deal was cheap pups (ie, Ceramic magnet blocks Under steel billets, Not real alnico billets)
Nothing wrong with that as some Ceramics can sound very good,,,you just have to be carefull not to wind to many turns as the ceramic will then get harsh.

Hope it helps you out a bit.  Phil.



« Last Edit: December 17, 2009, 04:25:07 AM by phatt »

kvandekrol

  • Regular
  • **
  • Posts: 36
  • Chip Points: 1
    • View Profile
I was also against active circuitry until I played this guitar! :) It may not work as well with all amps, but with mine on the right settings I can play clean with the guitar's volume on 6, have a very sweet overdrive on 8 and a heavy overdrive on 10. I'm not too picky about having vintage or classic tone so it's perfect for me.

You guys are the best. I'm still trying to process all the info from phatt's analysis, but I do have one question: What is it about the Elite's circuit that likes the 50k mid-boost pot, and what is different about the Clapton circuit that it needs a 250k? It sounds like if I were to only switch the pot from 250k to 50k with no other mods, it wouldn't make the EC sound more like the Elite, since there is some other stuff going on...

Speculation... What I think happened was this: the EC preamp was based off of the Elite's design. But the EC guitar used low-noice Lace pickups, eliminating the need for a hum-canceling single coil, so the dummy coil and the buffer were eliminated from the new EC circuit. Then in the late 90's Fender developed the Powerhouse based on the EC... but since it used American Standard pickups instead of Lace or one of the Noiseless models, it needed the dummy coil again in order for the active electronics to be usable. Rather than redesign the EC preamp to add a buffer, they just wired the dummy coil after the pickup selector and before the preamp. But the lack of an independently-buffered dummy coil changes the tonal characteristics of the pickups, because the signal goes into the preamp with a much higher impedance than if it was just one pickup. So it's not that the Elite and EC boards themselves are voiced different, but that the EC preamp board was not designed with a dummy coil in mind like the Powerhouse has. (Does any of this sound correct?)

So, short of buying an Elite preamp from eBay for the buffer, my other option is to install some Lace or Noiseless pickups and remove the dummy coil... but then, low-noise or noiseless pickups have a much higher impedance (usually 10-12k), so it probably wouldn't be much different than using a dummy coil...

I am coming to realize how ridiculous this all must sound! :) but I am fascinated by the sound of an Elite, and am very interested in reverse-engineering exactly where the tone comes from, so I really do appreciate all the help.

I don't have any pics of my own guitar, but here's a picture I found online awhile ago showing the underside of the pickguard. In this pic the dummy coil isn't mounted at all, but normally it is mounted between the pickup selector and the pots. For mine, though, I moved it right above the bridge pickup so it wouldn't be so cramped. I also attached the wiring diagram for the Powerhouse which shows the dummy coil in place.
« Last Edit: December 17, 2009, 05:59:53 PM by kvandekrol »

phatt

  • Legendary
  • ******
  • Posts: 2148
  • Chip Points: 262
    • View Profile
I was also against active circuitry until I played this guitar! :) It may not work as well with all amps, but with mine on the right settings I can play clean with the guitar's volume on 6, have a very sweet overdrive on 8 and a heavy overdrive on 10. I'm not too picky about having vintage or classic tone so it's perfect for me.


So, short of buying an Elite preamp from eBay for the buffer, my other option is to install some Lace or Noiseless pickups and remove the dummy coil... but then, low-noise or noiseless pickups have a much higher impedance (usually 10-12k), so it probably wouldn't be much different than using a dummy coil...

I am coming to realize how ridiculous this all must sound! :) but I am fascinated by the sound of an Elite, and am very interested in reverse-engineering exactly where the tone comes from, so I really do appreciate all the help.


If I had money to bet, then the *Elite* pups.
With or without Dummy load they are likely to be potted rail pups, overwound.
They are certainly very different that any other Fender pup.


You have the dummy coil and the circuit schematic why not just build the extra circuitry and run the dummy load like Elite?
Then you can make your own judgement.
No matter which way you wire it all,, the dummy is to up the winding which will fatten it up.

My old one time Elite owner mate just told me that the advertizing blab was to make it Fat like a humbucker,, which will subdue the hum somewhat by design. So Hum may not have been the reason for the dummy.

BTW,50 pot looks like it will work better,, less boost but more refined tone.
Phil.

kvandekrol

  • Regular
  • **
  • Posts: 36
  • Chip Points: 1
    • View Profile
BTW,50 pot looks like it will work better,, less boost but more refined tone.

One more request... If you saved a copy of the test file you used to generate the simulation for the 50k pot, is there any way you could run it again using the 250k (which is what I have now) so I can see the difference on the graph?

Thank you so much for your help so far.
« Last Edit: December 18, 2009, 05:36:29 PM by kvandekrol »

phatt

  • Legendary
  • ******
  • Posts: 2148
  • Chip Points: 262
    • View Profile
BTW,50 pot looks like it will work better,, less boost but more refined tone.

One more request... If you saved a copy of the test file you used to generate the simulation for the 50k pot, is there any way you could run it again using the 250k (which is what I have now) so I can see the difference on the graph?

Thank you so much for your help so far.
No worries,  The graph shows response of both 50k and 250k pot.

Note the use of 50k pot maintains an overall bigger output even with mid boost fully off.
That will make a sonic difference in the way you might percieve the outcome.
So my guess,,,The Elite circuit with 50k will always seem louder, more in your face.

(Note; take my earlier comment of 50kpot as a mistake)
Now that I've had more time to absorb it all the 250k pot (at full off) brings it down closer
to a *flat response*, to me that at least makes it possible to do a comparison with a normal strat.
Your ears will have to make a judgement on which is better.

If you want to go nuts with the circuit then C3, R6 and C13 are the ones to mess with.
Try C3= 15nF,, R6= 2k7,, C13= 2nF.
This will lower the boost point thus allowing more useful (audible) frequencies between 1kHz
and 4kHz to get through. There is only about 14 Db of boost at 3kHz but see that the rest of the bandwidth comes up also ,, hum ??? Head scratch.

So in effect there is only 4 Db of difference. IMHO not worth all the fuss.
Again only you will know what works best.
Remember there is very little benifit of boosting frequencies above 10kHz if anything you want to suppress them.

Between 6kHz and 10kHz will add some sibiliance to the upper harmonics but past 10kHz is just not audible through a normal guitar speaker, if it is audible you likely have a horrible amp speaker setup.
Note the vertical red line at 10kHz, so blot out anything above that line.

I've added the Mod curves after playing around with C3 and the other values to give you some idea of what will happen.
Have fun with it all, Phil.