freestompboxes.org

I need something to use at practice for vocals. We're using a solid state bass amp right now which appears to have too much gain for use with a mic. So, before I hunt down and purchase something like a PA head, I got to thinking about building something. I've built little bookshelf ipod amps using assembled tripath class D boards off of eBay, and they work great. Im wondering if anyone has experience with maybe doing something similar with a higher powered class D board (like 100 or 200W)? Or if anyone has any experience with putting together their own vocal amp in any capacity?

Do you want want balanced line Mic setup or just the cheap an nasty guitar jack?
Semi pro mixers are so cheap now it's hardly worth the effort of building a single channel unit.
Phil.

phatt wrote:Do you want want balanced line Mic setup or just the cheap an nasty guitar jack?

To be honest, I want anything that WORKS. Right now for practice i'm using a bass amp for vocals. This would be an Acoustic 370 amp, using a mic with an impedance transforming xlr-1/4" adapter on it. Basically its hard to get much volume out of it without feeding back, presumably because there's too much gain in the preamp for vocal applications. I would try using that as a power amp with a small mixer instead of the onboard preamp, but that amp doesnt have a slave-in.

Too MUCH gain? The pre-amp stage of a bass amp would have considerably less gain than a mic pre wouldn't it?

i would figure they'd be comparable, but i definitely cant turn the thing past 9'oclock on the dial without getting feedback, which tells me that maybe there's too much gain in the pre

All it tells you is that too much of the signal is getting back into the microphone. It might even be to do with the natural resonances of an instrument amplifier being more easily transferred back into a microphone than they would be into an instrument.

"Gain" is a much wider concept than just the gain stages of a pre-amp. The power amp is just "gain" too, it's just electronically convenient to do it separately.

Dr Tony Balls wrote:

phatt wrote:Do you want want balanced line Mic setup or just the cheap an nasty guitar jack?

To be honest, I want anything that WORKS. Right now for practice i'm using a bass amp for vocals. This would be an Acoustic 370 amp, using a mic with an impedance transforming xlr-1/4" adapter on it. Basically its hard to get much volume out of it without feeding back, presumably because there's too much gain in the preamp for vocal applications. I would try using that as a power amp with a small mixer instead of the onboard preamp, but that amp doesnt have a slave-in.

Q, Does the *impedance matching transformer* actually have a transformer?

Some of those units are just a cannon to 6.5 converter and folks assume it is a Z converter BUT some are NOT.
These will just complicate the issue even more. (Look inside for the actual Transformer)

The very moment you wire a balanced cable down to unbalanced (i.e. 6.5 guitar mono) you LOOSE BIG TIME.,,, I think it's a factor of -10Db.
So you have to turn the gain/volume up a lot further than needed to get the same energy transfer,,,,,and the result is often instant feedback.

Yes we would ALL like a quick fix but it ain't that simple.

Try inserting a 10k resistor straight across the input to the bass amp?
See if it allows more signal before FBack. If it sort of works then try playing with different R values,, even down to 1k.
(Caveat; this will only work *IF* the *impedance matching transformer* is a true Z matcher.)

There are other complex issues that may well be happening which are beyond my ability to explain.
Not knowing the situation,,,my best guess is:
Go get a decent mixer,, pad down the output to better match the Bass Amp input then you may get a better sound.

maybe Google Padding a mixer or similar.
Some of the old Yamaha mixer manuals had a whole section on making up simple resistive pads to Z match equipment.

Even with a mixer you may have to load down the Bass Amp input by a fair amount to stop Feedback.
Phil.

Oh forgot to mention,,, a huge wealth of pdf info on the *Jensen Tranny site*.
101 ways to interface PA equipment. Balanced and unbalanced,, transformer and transformer less interfacing.

I figure tiss better to get the info from the pros than a half educated chap like me.
Phil.

Awesome! Thanks for your help. And yes the adapter is a Z-matching adapter with transformer. I havent opened up, but thats what the package said when i bought it.

note....i work with this stuff all of the time**
** mic issuue**
use good low z mic ,,,sm 57,58,carvin cm60 . balance tranformer. make sure you understand PA spkr placement with relationship to mic. 8 mic being clearly off plane to the source . a bass amp is about as clean as they get. if you are too loud;there's your answer. bass,mid,treb, contols on the amp do the same thing on a PA mxr. even though i have had to go to "plan B" at times....ALWAYS use the right gear for the job. if you cant; then you are not ready. not being critical /just a fact. i do gear setup and repair here in Fallon NV, and i see goofy shit all the time! seen a guy blow up a nice vintage B.K.Butler RT2100 SS guitar amp playing a bass thru it...instead of taking 250$ to get a decent 300watt Behringer head! c'mon.
to be fair: i have used a 10' Carvin bass amp with a 25yr old sony 4chan mixer, 2 mics(with 1/4" adapters), guitar and bass to practice with!! easy ...most issues are with expectations. the amp on 8??? isn't that really loud? . mic sensativity is a big deal. turn amp down and sing like you mean it!...this is a demonstration of "signal to noise" ratio. ever see someonestart speaking into a mic and then back away once they hear themselves???..ya can't turn it up to compensate...the spkr just brought the signal to noise ratio right together!! LOL HUH..these are facts

donnyboy has entered the right idea.

Yes I totally agree ,,,IF I had a dollar for the amount of folks who setup a mic *INFRONT* of a speaker and wonder why they have FeedBack issues,,, heck I would be a very rich man. LOL.

If just practice or jam session then ALWAYS point the Amp straight back at you like a fold back speaker,,in a small gig that is all you need to do.

IF on the other hand you are being paid for the gig then you need front house setup ,, as well as Fold back.

Unless of course you have anti FB equipped gear then you can stand in front of speakers.

Anti FB does actually tend to rob the sound as it pulls very deep notches out.
Some like the freedom ,,, but I would rather work old school with smart placement and EQ tweaks.

Big gigs it matters little as you have paid the experts to do it all for you because you are very very famous,, winky.

Meantime those of us who just do it because we love music need to learn whatz happnin.

Phil.

Dr Tony Balls wrote:... Basically its hard to get much volume out of it without feeding back, ...

Could have a lot to do with the fact that bass amps usually have a characteristic "shape" (frequency response) even with the tone controls set at 12 o'clock. This normally takes the form of a mid or mid bass hump, if this coincides with any room resonances then the setup will feedback long before you have what you consider sufficient volume.

A graphic eq is very useful for eliminating feedback. Crank the volume until feedback just starts then find the frequency slider/s that reduces it. Drop this/these down until feedback stops. Then up the volume to the onset of feedback again and repeat . You are adjusting the frequency response of the amp to suit the room and speakers that you have.

This way you get the max volume, but not necessarily the best sound. For good vocal sound you need pretty good speaker response, something not all bass speakers have.

Also try and get correct impedance matching and speaker placement as already mentioned.