Looking to build a preamp

[Ayek]

So I got given an old 300w ss pa amp that my drummer dropped and was consequently too ascared to switch on again. Opened the thing up and it ain't too beaten up apart from a heatsink screw breaking off and the amp enclosure breaking apart. Anyway, I'm wanting to replace the preamp and would like some input from you people as to what to build. It's primarily gonna be used for bass. I've got +/- 62vdc in there, but if I gotta put a new power supply for the preamp in, so be it. Cheers.

[Hemisaurus]

How simple or complicated do you want?

DOD 250 preamp



Needs a 9V supply, and I'd add switching to those clipping diodes.

- or -



Or you could build in a Sansamp, or you could do a low voltage tube preamp, a Sun beta based preamp. There are bass preamp board kits available as well, MAJ used to do them.

[dunwichamps]

Acoustic 140 preamp?

[VOLVO)))]

Acoustic 140 preamp?

Yeah, that'd be baller. Two knobs for EQ and a volume.

[justinhedrick]

Acoustic 140 preamp?

Yeah, that'd be baller. Two knobs for EQ and a volume.

this.

[dunwichamps]

its a really basic preamp 2, I have seen the schematic. totally doable on turrets, and only need unipolar +75V supply so +60 or so would be fine 2.

[VOLVO)))]

its a really basic preamp 2, I have seen the schematic. totally doable on turrets, and only need unipolar +75V supply so +60 or so would be fine 2.

There's pictures of it in the tech thread.

[dunwichamps]

its a really basic preamp 2, I have seen the schematic. totally doable on turrets, and only need unipolar +75V supply so +60 or so would be fine 2.

There's pictures of it in the tech thread.

oh nice. I actually did a turret layout for someone already before so I got it down as well

[Ayek]

I'll check that out. Quite a while back I built that modernised op-amp based 360+ preamp that's been floating around the innerweb, but it didn't quite do it for me. Though maybe it was something I done, a few things I'm not proud of occurred on that build

[Ayek]

Ok, so it looks like I'll have to look abroad in order to source the 1.9 Hy inductor and the 2N4248 and MPSA09 transistors. Where's a good supplier for such items that ships worldwide? My sources within my country don't have them.

[spookstrickland]

If it were me I would use discrete germanium transistors instead of op amps and I would just do some kind of cascading rangemaster or Lpb type design but leave the cap of the emitter leg.  five or six of those in a row with a couple pots in the middle for pre and post gain and you would have a fucking monster!

[dunwichamps]

Ok, so it looks like I'll have to look abroad in order to source the 1.9 Hy inductor and the 2N4248 and MPSA09 transistors. Where's a good supplier for such items that ships worldwide? My sources within my country don't have them.

2SA970 and 2SA1137 are good enough equivalents for the 2n4248

http://www.talonix.com/shop/#.UEoB-ZaO58E

That shop has the equivalents and the MPSA09

The 1.9H inductor is more difficult

One solution is to buy two 1H inductors and series them

You could request free samples from Wilco, I have done that before

http://www.wilcocorp.com/catalog/series.cfm?series=BSS

Pick the last one on that list.

an ~2H inductors is not easy to get in small sizes. Theres plenty of big 2H chokes I can use for amps but they are designed to be large to handle a lot of power and they arent super cheap, something like 20-30 bucks for 1. Also they take up space!

[Hemisaurus]

Ok, so it looks like I'll have to look abroad in order to source the 1.9 Hy inductor and the 2N4248 and MPSA09 transistors. Where's a good supplier for such items that ships worldwide? My sources within my country don't have them.

http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search

2N4248 = NTE159
MPSA09 = NTE199

unless you prefer the smell of cork

[Ayek]

I'm down with using substitutions, for sure. I'll look into wilco, my usual suppliers only have up to about 100mH inductors

[Hemisaurus]

You can connect inductors in series to increase the inductance (opposite of capacitors), I seem to recall RG Keen's analysis of the Wah pedal said that the Radioshack audio transformer (primary), was a viable substitute for a wah inductor, they come in at .66H so either three wah pedal inductors, or three of those $3 jobs from RS would get you in the ballpark. You can also look at small coupling transformers specs. whatever you have available locally.

If you have some kind of bridge that can measure inductance, maybe in a school/college lab, you could measure whatever you have to hand or try winding your own.

[dunwichamps]

yea 1H or 1.5H is probably sufficient honestly for the filtering. Its probably easier to find 0.5H stuff around than 1+ H inductors

[Hemisaurus]

Having not even looked at the schematic I'll say. Couldn't you just change the tone-stack

At it's almost simplest you need two gain stages and a tone-stack, one to buffer the input and put it at a level for the tone stack, and one to make up the gain lost in the tone stack and make it a suitable level for the outside world.

Like spook said, a gain stage, it's an LPB, it's a transistor.

[dunwichamps]

Having not even looked at the schematic I'll say. Couldn't you just change the tone-stack

At it's almost simplest you need two gain stages and a tone-stack, one to buffer the input and put it at a level for the tone stack, and one to make up the gain lost in the tone stack and make it a suitable level for the outside world.

Like spook said, a gain stage, it's an LPB, it's a transistor.

You could but then its not a Acoustic 140/150, its something else, whether that is good or bad is up to you but the transistor design was based around the EQ.

[Hemisaurus]

Well it's not really an Acoustic 140/150 as the power supply is wrong, and the power stage is different

It's kind of a situation reversal, most equipment was designed around what was convenient for the builder, Marshall took the Bassman and substitutes local tubes, etc. etc. Nowadays people try and source the exact same part, no matter where it has to come from, as if it's some kind of holy grail.

In this particular instance though, I'm sure an inductor could be found, but you don't have to

[dunwichamps]

going from 2H to 1H moves the low end control higher up on the crossover from say 750HZ to 1K, its a shelving filter so thats probably fine

[Ayek]

After doing a bit of hunting and such, I think I might as well have a go at the JFET preamp Hemi posted. The description on the dude's site make it appear to be something that might do the trick. Seems like a lot of the older circuits use obsolete and/or hard to find components that'll take more effort to rectify than my short attention span will allow. I'll let y'all know what happens.

[Hemisaurus]

That was just a random Google, but if you're going the FET route, here's a couple of artices on FET based tube preamp copies, where a tube preamp is recreated with FET's.

http://www.runoffgroove.com/ginger.html



Ampeg Fliptop.

[moose23]

Have a look at muamps too while your at it. Stuff like Catalinbread SFT etc is based on muamps.

[Ayek]

Right, so I thought I'd make the Acoustic 140 preamp but with a bax type eq instead. Could someone please check my work? I'm not 100% about my placement of C108 specifically

before:


modified:


I might also put a 560pF mid boost cap parallel with the 330pF either on a switch or I heard a 1M pot would work as well

[dunwichamps]

that looks good. If your curious to see the response download Duncans Tone Stack Calc, input the values then sweep the pots. I suggest converting the 180k resistor into a pot wired as a rheostat, its known as the Shift control (from ValveWizard and I use them all the time). It causes an interplay between the crossover between the bass and treble, almost has this nice mid range function.