Hey what's up everybody?
I saw a video of a homebuilt randall preamp and thought that looks easy. So I copied the schematic and laid it out on a breadboard but now when I hook it up it just buzzes.
I know I must've done something wrong but I'm just wondering if anyone with any experience can point me in the right direction ie what to check/test?
I didn't include the tonestack curcuit I'm thinking maybe omitted some necessary detail in there?
I am plugging my schecter into it then running the signal out to a cheesy estbean 10 watt.
1) post the schematic you used
2) grab pencil and paper and draw what you you actually built
3) post a couple close up pictures of what you built.
In theory all 3 should match ... in practice there is some difference, ... or it should work.
Only after we have that we can suggest some voltage readings to check, also some signal tracing.
Oops sorry about that let's see
Here's the 2 schematics I used. when I get home I'll take a picture of my pen drawing of it and what I actually did. also there's the first picture again but I marked what I left out.
As you have deleted the tone then you need to *Decouple* the circuit from DC,, so a 2uf<10uF cap at the output will help.
Likely other stuff needs work but keep at it it should work. :tu:
Phil.
Here's my tracing and what I actually did (with the suggested addition of a 10 uf cap at the end.
Also I forgot to mention the schematic calls for 2n5484 transistors but all I had on hand were 4401s that could be my whole problem.
Tonight or tomorrow I'll draw what I actually built including series resistor and parallel caps because that's most likely where my problem lies
Hi Charlie,,
Bottom pic;
Looks like you have added a 10k R/C filter with 3 electro Caps to common,, (which is a good idea)
but you have a black wire dead shorting the 1Meg input R ???
That black wire should be going to that 33k resistor ??
(which BTW is banded Red/Red/Orange which is 22k,,33k would be Orange/Orange/Orange).
If you have a Meter you can check that you have DC voltages at the right places in the circuit.
If you want to check your wiring just turn the BB around 180' and check the BB against the schematic,, again.
This forces your brain to think it through in a different way and in doing so you often find what you missed. good training,, 8|
Phil.
Ed; I should add that without the tone control don't expect great things as the tone is part of the trick. :-X
Quote from: Charliegriffin on August 12, 2016, 04:56:57 AMAlso I forgot to mention the schematic calls for 2n5484 transistors but all I had on hand were 4401s that could be my whole problem.
Do you mean 2N4401 ? That's an NPN bipolar transistor. The 2N5484 is an N-channel JFET.
Hi phil thanks again already I've found some things by just trying to check myself for this reply
Kind of a crammed BB there I got some resistors in series when I get some time today I'll try to draw out exactly what I did but I'll try to give it my best description for now.
Let's see the input goes into I believe a 56k resistor in series with a 10 to get me closer to 68k then into the little cap (.02 I think) from there my schematic said 250 cap to ground (*found a discrepancy there I'll elaborate after*) and 1m resistor to ground. I didn't have a 250 uf cap so I paralleled 2 100s and a 47 which I put on their own little rail or whatever with the black wire connecting them to the .02 cap the 1m resistor and the first transistor.
The 33k resistor is in fact, you are correct, a 22k but it is in series with a 10k since I couldn't find an actual 33k. As I have it they are running from +24 rail to the collector leg of the first transistor I'm pretty sure I get terms switched around in my head a lot
As far as the tonestack I plan to make that next I only cut it because I didn't have the right pots I was hoping they would magically appear while I work this out haha.
Now in the course of writing this reply I found 2 things in my pictures. The first I kind of mentioned was my 250 uf pot. My actual randall whole amp schematic seems to imply 250 uf but on the preamp from hell page it looks like it says 250 pf so I'll have to try Google and see what I can find
Next thing I saw is maybe more significant
The emmitter leg of the first transistor (q5) is supposed to have a 2.2k (r22) to common but I hit the wrong hole (phrasing BOOM! Haha) I'll fix that when I get home and then 180 flip the bb and run through it reverse order that's a really good idea
Quote from: Loudthud on August 12, 2016, 04:20:09 PM
Quote from: Charliegriffin on August 12, 2016, 04:56:57 AMAlso I forgot to mention the schematic calls for 2n5484 transistors but all I had on hand were 4401s that could be my whole problem.
Do you mean 2N4401 ? That's an NPN bipolar transistor. The 2N5484 is an N-channel JFET.
Oh man. I must have misread I thought I read something that said they were the same type. So do you think it's safe to say that's probably my problem? Find some n channel jfets with similar/ higher ratings? Thanks for pointing that out to me Im not too good at details apparently hah
That cap to ground at the input is 250pF NOT 250uF. :o
Good catch by *Loudthud* yeah BJT won't work,, you need Fets. :tu:
Phil.
Alright thanks guys now I know I got to order the some jfets today. So I'll have to pUT this project on hold while I wait for those.
While I'm at it I'll get the pots I need for the tone curcuit and some extra pieces to have
I'll keep you updated next time there's any news
Ok so once again thank you loudthud. I have replaced my bjts with some mmbf5484s and am now getting sound from my guitar through the preamp and out of my amp. One of my solder joints broke right before I started try ed to test it so I just set the two pieces together so I could test it and finish for the night. Tomorrow I will fix that and add the tone stack then post an update.
Alright I've got my BB all put back together I've been working late all week so I only get about a half hour to an hour a night to mess around and by then I'm usually pretty tired, but anyway I got the tonestack in and finalized last night and got to test it out for a quick second. I've definitely got signal to the the speaker now but there's a lot of buzz and hum. I can reduce it by touching my guitar strings and even a little more if I touch a knob on the preamp not very much but slightly. I'm sure there's some ground looping and interference due to sloppiness and the fact that it sitting open in a breadboard and I'm fairly certain I've got something wrong still some capacitor value or maybe a misplaced component or who knows. So tonight I'll compare the actual circuit to the schematic see where I may have messed up. But now the new questions I have are
1, is it possible that if I put the BB into an enclosure would that cut much of the noise or would it be a pretty unnoticeable difference?
2, reading threads and Wikipedia and other Google sources it looks like a lot of people say the preamp can be grounded from a rail without worrying about ground loop problems is that correct or am I misunderstanding ?
3, would to much amps from the power supply cause a problem? My PS is 24v 14.something amps.
And finally 4, can anyone recommend a simple chipamp to use as a power amp? I'd like to keep it light, say somewhere around the 5-20 watt range I don't have a place currently to play very loud and the only decent speaker I have is pretty small until I can afford to buy a couple new-2-me. I also would like to power it with the same 24v PS I'm running the preamp off
Is the power supply a switchmode plastic unit from a computer or similar?
If so be warned some are very noise prone as they were not meant for Audio circuits.
It is probably similar it's actually 2 xbox 360 power supplies running in series and they are plastic. If I ran it off of batteries is it possible that would take out the power supply noise or would it make similar noise still? Also since I don't have any 12 or 24v batteries would it be inadvisable to try and run it off a lower voltage like say 18v from 2 9v in series?
Looks like they are 12VDC at 12Amps. plenty of current
Not sure about using them stacked in series for double the Voltage but obviously it's working for you.
You may find dirt cheap computer power packs at your local recycle dump shop or Salvo's and other op shops. I've picked up several switch mode power units at my local recycle outlet.
I Picked up one this week for $2. 9VDC at 2Amps,,,just needed a mains lead.
Perfect as I was wanting a spare power unit for my pedals but it does have a noise issue,, (read more below)
I've found it's a lottery when using S-mode supplies for Audio as most are real noise makers. :grr :trouble These use High speed switching circuits and introduce a lot of RF noise which requires a fair bit of filtering otherwise that high pitched whine ends up coming through you speaker.
I ran into a problem a while back while trying to plug my korg Keyboard into a big PA system. The Keyboard is powered by a switchmode unit and although the hi freq whine is barely audible through the keyboards on-board speakers once you DI the line out to a full bandwidth,,i.e. 1,000 Watt PA the hi frequency whine is clearly heard. Needless to say I was very pissed off as it stuffed up my whole gig that day. :grr
I've recently been looking into how to circumvent this problem in future and found good advice here; http://ka7oei.blogspot.com.au/2014/08/completely-containing-switching-power.html
This chap explains the problem very well and shows how to make your dirty switching power supply clean. <3) <3) <3) I'm experimenting with some of those ideas right now to see how well it cleans up and so far I've cleaned up the high freq whine,,, just need to work out how to install it all in a neat tidy box. If it all works out I'll post the results. :tu:
Getting back to your Q4/ regarding a low wattage power amp chip,, Easy to get 10Watts from an LM1875 and I think they may even run on a single supply. Others here may have even better options for you to explore.
Phil.
I'll check around my local recycling salvage next time I take my trash that way (I usually use the dump nearer my work which doesn't take any electronics) see what I can find. Thanks for that link that's a very interesting read and a useful couple of curcuit definately bookmark material there. If I mimic the process that guy took with a couple steps I could potentially turn what I have into a more permanent solution. My original intent was to salvage or buy a 24v or higher transformer eventually and build a power supply for it but maybe I'll skip that step.
That lm1875 looks like the best option I've looked at so far I'll probably tack a few on to my next order to experiment with.
After looking overy my breadboard I found a couple capacitors that were way off replacing them cut actually quite a bit of the higher pitch whine and even some buzz. as long as there is notes coming through the speaker it sounds good just the silence isn't totally silent
I also tried running it using 2 9v batteries connected in series to make 18v it had about the same amount or buzziness either way I don't know if that means much. If I put my hand pretty close over the curcuit I can cut the static-y kind of hissy bits of the noise down to where it's a pretty dull even hum I assume that's basically a preview of what I can expect to change when it's mounted in a grounded chassis? I don't know maybe not
Another thing I'm starting to wonder is how much noise is coming from my cheap (and pretty far from new) cables or the amp I'm hooking up to or even my guitar that my be the next test I make.
Anyway thanks for the help and interest I'll come back with any updates