4 x KT88 Bass amp ?

[Reeltarded]

Silly!!

[Smokebreak]

So it looks like I'm finally going to get to build a bass amp for someone, though it's not going to be a 200W deal. Based on suggestions in this thread, it looks like we're going with a 25W SE head.
The goals:
-Plug into speaker cab for home use/practice
-Plug into cab and take a balanced xlr line out, or unbalanced, to a Class D amp
-No plug into cab and take balanced or unbalanced to Class D

I'm not terribly concerned about the preamp at this point, I just want to make sure I have the above options covered.

I've attached a proposed drawing.

Is that the best way to do the purely resistive dummy load switching? Can I just bolt one of the aluminum dealios to the chassis? heat sink?

How would I go about determining the proper values for the voltage divider off the secondary, for the line out?

I'm also considering just a pre-amp line out, before the PI.
Can I just tap out right before the PI, and still have the above 3 options work(with the preamp line out used, instead of the OT line out)? Or would I need some sort of loop to take a preamp out, and still have the amp's output section functioning, being able to use a cab directly from the speaker jack?

[xtian]

I had good luck with a faux (meaning NOT) balanced XLR output, like the attached.

[JMFahey]

basically a good idea but please be aware that the actual construction does not match the schematic.

That's a 1k2 1/4W resistor, not 47X .

The proper value is 47K for 10:1 attenuation; 1k2 will let some hairy , mixer input killing voltages reach that XLR.

[Smokebreak]

Is 10:1 the attenuation I'm looking for? That's what I'm trying to figure out. If so, why not 100K/1M, or 100r/1K?

[xtian]

Well, the value came from practical experimentation. We could not get hot enough output with the 47K in place.

[Meat&Beer]

If you're looking for a balanced XLR line out, this is what I've used. It's all the way at the bottom of the page, 4 bucks!

https://taweber.powweb.com/store/magnetic.htm

[Meat&Beer]

Also just came across this if you haven't seen this particular one yet. Haven't tried it. Just for reference/ideas.

[Smokebreak]

If you're looking for a balanced XLR line out, this is what I've used. It's all the way at the bottom of the page, 4 bucks!

https://taweber.powweb.com/store/magnetic.htm

Thanks! They also show the complete kit but note the different size pot. From the looks of things it sounds like experimentation is in order. I'm hoping to grasp some concepts as to why these values are what they are: [img632]https://taweber.powweb.com/store/balun_schem.jpg[/img]

[JMFahey]

Is 10:1 the attenuation I'm looking for?

At least !!!!!!
You are attenuating speaker out voltage.

For reference, 50W/8 ohms is 20V RMS

Typical power amp sensitivity is 1V RMS so 10:1 gives you nominal 2V RMS max , which can be padded down to 1V RMS with the pot.

Now a typical XLR mixer input is meant for a microphone, I wouldn't send it much more than 50 or 100mV ... do the Math.

Those 1k2 in series with a 5k pot will yield some 15V RMS in the example I mentioned, enough to destroy any mixer input.

Code: Select all

why not 100K/1M, or 100r/1K?

It's the other way round: in series the larger value; to ground the lower one.

If you are driving a Mic input, those are low impedance (around 600 ohms) and resistors used in the attenuator must roughly be in that area or lower.

One value set which does not even require a pot because it's in the comfortable zone is 10k/100 ohms , with 10 k from speaker hot to XLR pin 2 ; 100 ohms from pin 2 to pins 1 and 3 which are grounded.

Signal out will be in the area of 15/35mV RMS depending on amp, which any mixer will love.

We could not get hot enough output with the 47K in place.

See above about suggested level and impedance values, but in any case 1k2 is WAY too low, so I guess there was a wiring / cable / connector mistake somewhere along the path ... or simply the mixer input was also using a "faux balanced in" and your and their "cheat" did not match.

[martin manning]

Nice explanation, Juan!

[Smokebreak]

Yes! Thank you very much Juan.

So if I'm looking at a SE KT88, lets call it:

20W @ 8ohms = ~12V RMS
10:1 gives 1.2 V RMS
240:1 gives 50mV for mixer

So if I've got this correct... 10K to 1K pot dialed down to ~40 ohms satisfies the mixer, and dialed down to ~800 ohms satisfies the power amp.

Is 1.2V RMS enough "headroom", or maybe back off of the 10K a hair?

[ampfab]

Hey m fowler, what b+ voltage are getting from the 378cx power transformer?

[M Fowler]

Hey m fowler, what b+ voltage are getting from the 378cx power transformer?

I had to shelve the project due to repair work and family problems. Hopefully I will be able to wire it up soon. Helping to raise my grandkids and they don't let me work in the basement and I can't concentrate with the cuties around.

Mark

[ampfab]

Hey m fowler, what b+ voltage are getting from the 378cx power transformer?

I had to shelve the project due to repair work and family problems. Hopefully I will be able to wire it up soon. Helping to raise my grandkids and they don't let me work in the basement and I can't concentrate with the cuties around.

Mark

No worries, thanks
I assume you'll have about 560v