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Ideas anyone?

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6/17/2005 4:05 PM
Mr. Brokenamp	Ideas anyone? Any ideas for mods to help out the low end in a power amp? Increased filtering, low end boost switch etc... All with the intent of increasing the low end and tightening it up.

6/17/2005 7:25 PM

Pi filtered B+. For a choke I like the Triad C-40X. Available at Allied. 320mH @600mA 10 ohm. Takes a big cap on the output but the ripple will be unbelievably low, like a 1V sine wave. You'll be able to see a rectified version of the signal on the B+ at low frequencies. Value of the second cap influencies the low end. Value of the first cap controls amount of sag. Let the coupling caps to the output tubes control the LF rolloff. It's easy to get too much low end.

6/17/2005 7:46 PM

Not to nit pick...I believe a choke is most effective when it's power rating matches the circuit it's going in. You actually get fewer henries than stated if the choke is over rated by too much. If you mean to put the choke in front of the power supply in a typical 100w guitar amp then the one you suggest is fine. But if your placing it between the plate and screen voltages as is normally seen then a 600ma 320mH 10 ohm choke will probably act more like a 10 ohm resistor. Think about it. If you take the whole arrangement to an extreme and use copper wire the size of rope and a core the size of a steamer trunk, a little bitty 300ma or so isn't going to create much inductance.

JM2C. Not trying to start a flame

Just some helpful info I gathered somewhere along the way.

Peace

Chuck

6/18/2005 3:43 PM

I always thaught that increasing the current would reduce the inductance as the core gets closer to saturation (and then the inductance drops off like a brick when you saturate it, which is why there are air gaps in SE trannies and chokes to stave off saturation). I seem to remember having some "swinging choke" around here that has it's stated inductance when the circuit is operating, and some freaking high iductance when the load isn't drawing any current.

6/18/2005 5:18 PM

The fundamental nature of my suggestion is to increase the capacitance at the node where the output transformer is connected. That's what helps the bass. Without a choke, the peak currents in the transformer and rectifier get too high. With a choke there is practically no limit on the output side capacitor.

In a bass amp I'm currently working on I used one amp fast recovery diodes, two 30uF @ 600V on the input side, my favorite choke, and on the output side a 330uF @ 450V in series with a 1000uF @ 200V. The voltage divider resistors are set up to give 150V across the 1000uF with 600V input. It's a little hairy because of the differing leakage currents so next time I'll probably go with two 470uF in series. At full power (1KHz clipped square wave) the 60Hz ripple is less than 1Vp-p. This is running a pair of KT88's with an output about 80 watts. The driver stage didn't perform the way I wanted so I only played through this amp for a half hour or so. The bass was actually tighter than I would like.

A swinging choke works great for choke input so that the keep alive current doesn't get too high requiring a 25 or 50 watt bleeder resistor. I've never really tried this.

6/18/2005 10:51 PM
Chuck	Gottcha, I has too busy counting Henries. Chuck

6/19/2005 1:46 PM

I'm not sure. I think I may have read it here and the explaination made sense to me. It could be that no load and a small load act very differently too. of course the inductance must drop off at saturation so perhaps there is a curve to it. I just know that I read somewhere that if you use an oversized choke then you aren't getting all your henries.

I hate to be short of henries. I like to keep one on the porch for greeting guests and another for cooking and cleaning

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