Chipamp preamp with blend pot

[dogbox]

Hi guys, just getting back into electronics after a long break. I'm putting together a small practice amp, using panasonic AN7523 chip. Power amp works well, and is safe to use for novices like myself, as it runs 8v and outputs 4 w into 8 ohm speaker...more if you power from 12v supply (closer to 6w). So far tried various types of preamp circuits ... built Joe Davisson vulcan pre, vibin' champ, simple tillman, fetzer. all work ok  - but lacking in features that i require..i.e clean, distortion. Lately have been playing around with an idea from this circuit by Len Galasso found http://www.generalguitargadgets.com/wp-content/uploads/practiceman.gif. I've "frankensteined" (designed is much too strong a term haha) a version that has a mxr clean boost for the clean channel, and boss sd-1 as the "dirt" section. All woking well. with with one exception, the blend pot does not really work as I imagined - as you turn the pot..it goes from sparkly clean, to a volume drop at about the middle of the pot's range and tiny bit of blending of the dirt signal - then as u keep turning immediately to volume increase and straight overdrive. What I'm looking for is nice blend from the clean to the overdrive. Can anyone suggest a way a better way to achieve a proper blend between the 2 signals???
cheers
db

[Phoenix]

Sounds like you might have a phasing issue between the "overdrive" and "clean" channels. If one is inverted in respect to the other, you would get exactly the symptoms you describe. If this is the case, then an inverting stage on either of the channels should rectify it.
Impossible to say for certain this is your problem however without seeing what you've "Frankensteined".

[dogbox]

Hi Pheonix,
OK..I have the "OD" signal going through the first 3 stages of sd-1 Transistor input/buffer, then next 2 inverting opamp stages...

The clean signal is a single inverting stage mxr micropamp...output here meets up with the output of the 2nd opamp in the sd-1 circuit...

I'm not sure if the sd-1 transistor input stage inverts or not?? If it does, then I have 4 x inverting stages, then I'm guessing that phase is not the issue. Good to know that phase can be an issue.

If it is due to phase, then I add an inverting after the mxr clean..would it be a simply unity gain inverting stage?
This is as far as I have gotten so far, i was planning on adding the rest of Practiceman after the summing amp stage, which is a baxandall 3 band eq, then a final output stage..both inverting.

Does it matter at all to the power amp whether the signal is inverted or not??? Is it good form, to always have the signal in phase (non-inverting)??

Thanks for your help
cheers
db

[mth5044]

The power amp probably won't care, but when you try to mix two out of phase things, you get issues. How are you doing the summing?

[antonis]

I'm not sure if the sd-1 transistor input stage inverts or not??

Neither us without a schematic..

If signal comes from that transistor's Emitter/Source, it is NOT inverted..
        //          //                 //       Collector/Drain,  it IS inverted..


edit:
<I have the "OD" signal going through the first 3 stages of sd-1 Transistor input/buffer, then next 2 inverting opamp stages.>

I the above stands, it isn't inverted..!!

[SteveJF]

Hi guys, dogbox is dead:) I killed him off. I opened that account years ago, and due to inactivity I needed to reset the password, but thats going to an old defunct email account (5 isp's ago)..and there doesn't seem to be an email contact for an admin to reset my password. Anyway I'm back, with a new account and under this name "SteveJF".

Yes I sorted this out and it was a phase issue..thanks for the assist. I got the blend pot working. Theres a large amount of variation because there 2 volume pots..one from the SD-1, and one from the mxr microamp...depending on which one is higher, it effects the the blend..interesting but not practical... I will just put in set resistor for SD-1 on max, and be done with it.

In the process of doing up a schematic, as it is hard to keep track of where i'm at. Gonna need some more help in adding active sallen key filter section..so i think I will get my schematic ready and start a new post.
thanks

[antonis]

dogbox is dead

Long live to SteveJF, then.. 

I killed him off

Well done...!! 
(although you leave us to decide if you've commited murder or suicide..)

[duck_arse]

welcome to the forum, SteveJF.