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Old 30th September 2005, 03:58 PM   #1
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Default Brian's LM4780 for Bass Guitar

It was a nice experience to build BrianGT's parralel-style LM4780 kit. I tried a point-to-point one first, so Brian's was a groove, and I learned a few things in the process.

Something I'd like to do with them is to replace the power amp of an old Polytone Custom Bass amp, since the original one was mangled. It has a center tapped power supply with -30-0-30v for the power amp, and -24-0-24 for the preamp. There's a pair of big old screw terminal 4000 uf capacitors on it. One speakerbox I have has an 8-ohm Eminence deltalite 10" bass guitar speaker in it. My bass has an onboard active preamp.

I found that I can plug a cd player straight into the Brian amp and it sounds great, with lots of volume. Or, I can plug the cd player into the Polytone preamp and Brian power amp. That also puts out lots of volume, and no paticular distortion.

But if I plug my bass straight into the Brian amp, it clips at fairly low volume, and the most when I strike the lowest strings. I guess this is because the fatter string makes the pickups put out more juice, but I didn't measure any difference at the bass's output, assuming I'm doing it right.

I get the same thing when plugging the bass into the Polytone, or using my two Ampeg's preamp-outs, or my Eden, or my PAIA mic preamp. All of these sound very good, except I can't turn up very loud. The bass's impedance measured at the end of the guitar cord, is about 23k ohms.

The speaker cone doesn't travel far, when playing a CD through it loudly. But the bass guitar makes it jump right out there, especially on a low E ( about 41 hz ).

Am I using all the amp's power, or can I make something that will fix the problem?

Right now, I'm making one of Decibal Dungeon's buffer circuits.

Pete
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Old 30th September 2005, 05:33 PM   #2
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I recently "fixed" an amplifier that was clipping the hell out of the low notes on my piano by installing a beefier power supply -- same voltage, more current available. Every time I hit a loud note on lower 1.5 octaves, it would clip HARSHLY and actually take a half second or so to recover! [crappy amp!!!] Keyboard goes down to low A.. in the 30Hz neighbourhood.. a little lower than a typical 4-string bass.

Your problem is probably one of two things (in my newbie opinion):

1. You can't deliver enough power
2. You're exceeding either the input or output power specs for the chip.

If you're using a 30-0-30 transformer, I'm expecting you to get about +/-42 VDC rails. That's exactly the maximum rating for an LM3886; I'd think the 4780 would be the same. Be careful with your heat sinking!

Anyhow. Assuming that your power supply can deliver a good 5 amps, you should be able to output a crapload of power with this chip. The LM3886 would be able to put out over 50W into eight ohms. How loud is the amp before it clips? Even with bass notes, 50W is LOUD, adn the 4780 should be able to do twice or four times that (depending on you're using a bridge or parallel configuration).

(Hey, what is the nominal impendance of your speaker array?)

If you ARE overdriving the amp and have enough power, I'd suggest turning down the gain on the amplifiers. Remember, it must be at LEAST 10 for the amp to be stable. I think Brian's kits are all at 30.

Attenuating the input would be another option, but due to the nature of guitar signals, I would think that you'd get better results leaving the input nice and hot and turning down the gain.

Wes
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Old 30th September 2005, 06:38 PM   #3
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Thanks Wes,

The power rails are -30 and +30v, and the speakers are 8-ohm, although I also have a 4-ohm cabinet.

I'm inclined to think the power supply isn't the problem, but I'm not tossing out that idea. It's the same PS setup as the usual Gainclone ones, and it was originally powering a 60-watt transistor power amp.

I don't know the VA rating of the transformer, but it probably weighs about 6-7 lbs. The original amp is an early version Polytone Custom Bass amp.

Knob combinations on any of my preamps, or the bass, don't get me more volume without the clipping.

I suspect this is normal behavior for the setup, and can be solved through measurements and formulas, but I'm not sure where to start.

Pete
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Old 3rd October 2005, 12:05 PM   #4
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> I suspect this is normal behavior for the setup, and can be solved through measurements and formulas,
> but I'm not sure where to start.

Tell me about it!!!

If you're getting 30VDC rails, either your transformer isn't putting out 30VAC, or there is something horribly wrong with your powersupply (or you live in a region where the laws of physics are different).

Oh -- I just realized -- are you running a REGULATED power supply? If so, why, and why keep the 30V rails? I'd shoot for 35.

Anyhow. That's really irrelevant.

Try attenuating the input (volume knob) or lowering the gain (see chip specs). One of the two is bound to help.

Does the bass have an active pickup? What kind of voltage does a bass put out? CD players put out something in the neighbourhood of 2Vpp FWIW. Do you have access to a 'scope to look at the waveform? What kind of pickup do you have?

I've never hooked a bass straight into a board or anything, we always plug the basses in through bass amps' outputs. Maybe there is a reason for that?

Wes
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Old 7th October 2005, 04:28 PM   #5
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I made up a different power supply, non-regulated, with more capacitance. It did improve the bass-clipping problem a little, and the low-end sounds better.

The change makes it look like I have the gain problem described in Brian's LM3875 manual.

The rails are 33v now, and the heatsink does get somewhat warmer.

Wes, you didn't read what I wrote before you posted, but you're the only one that answered.

So, thanks!

Pete
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Old 7th October 2005, 04:32 PM   #6
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How big are your heatsinks? Any pics of the setup?

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Brian
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Old 7th October 2005, 05:33 PM   #7
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My heatsink is still in crude-mode, but it has one that's 3"x2.5"x1" stuck face-to-face with one that's 3x4x2".
I was just running the radio through into the amp, and the heatsink was only a little warm.

It got much warmer yesterday, while I played bass guitar into a bass amp, and fed the preamp-out into the LM4780. I could turn the bass-amp's volume all the way up, without getting much volume. But the gain control got kind of touchy.

I had slapped together the newer power supply, to try out a new transformer. I used the rectifier board with all the diodes in, and no jumpers, but used only one side of it. I was thinking I'd use the other half for a different voltage, for a preamp, trying to see if the rectifier board would be OK with that.

It will take me a while to get a picture.

Thanks,
Pete
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Old 8th October 2005, 01:04 AM   #8
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> Wes, you didn't read what I wrote before you posted, but you're the
> only one that answered.

I actually have an excuse, I've been extremely sick all week. It's making it difficult to absorb information. But I *need* to do things like post on here (and work on my amp) to prevent myself from losing sanity. I'm not used to not being at work!

> So, thanks!

You're welcome!

I'm actually quite interested in your project, as I'm building an LM3886-based keyboard practice amp. Keys and bass have a lot of the same issues with low notes.

If you get your problem to ameliorate, I'd love to hear what you're doing.

Wes
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Old 9th October 2005, 04:25 AM   #9
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I found a grounding booboo, and I think it cleared up some of my clipping prob ( see the first post). I can play loud now, if I go through my PAIA tube mic preamp, into 4 ohm speakers, although it sould be smoothing over some clipping with the tube fuzziness. The DC offset is 40mv.


For the VA rating of a transformer, is it figured just the secondary's AC volts x amps ? As in watts? ( Don't laugh, I'm sensative).

My transformer's secondaries I'm using are -24-0-24, and it's only 2 amps. Does that mean it's va rating is 96va? Not enough amps for what I'm doing? Is it like I need the transformer to have about 5 amps ( like Wes mentioned earlier) to get a rating of 240va?

Useful for driving my 300-watt rms bass speakers louder without clipping?

I was considering, what if I made Zang's bridging board and tried to make my amps switch back and forth from stereo to bridged mono. So I'm wondering what kind of transformer that will take.
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Old 9th October 2005, 08:05 AM   #10
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Yo, Pete!

As I understand it, VA is actually Volts * Amps * Power Factor -- but Power Factor is usually very close to 1, so for diyaudio purposes, you can interchange VA and W.

96VA is definately not enough for what you're doing.

As for running in bridge mode, I'm considering the same thing with my amp ... But since there is even more output power (up to 4x), you will need more input power.

Of course, many people calculate their power requirements and divide by two for the transformer VA rating, saying "Hey, it's music". (See recent thread in Solid State forum). But both our instruments are able to generate a LOT of low frequency energy, so I'm not convinced that's a good way to go.

Unless you play exclusively pizzicato bass.

Let's play with NatSemi's spreadsheet: http://www.national.com/appinfo/audi...gn_Guide15.xls

For a single IC in parallel -- Pd is 49W, Po is 86W. So, for a two-channel amp, I would specify 2 * (49 + 86) = 270VA per channel, 540VA total.

If you want to bridge them, I would specify 2 * 49 + 4 * 86 = 442VA per channel.

Of course, I have be accused of way over-specifying transformers. *shrug*. I want to make sure that when I hit a ten-second-sine-wave at 31Hz, full blast, that my amp doesn't freak out! Hell, even playing low piano notes loudly through a typical Hi-Fi is enough to put it into convultions.

Conclusion: How about two of these babies? -- http://www.partsexpress.com/pe/showd...number=122-640

at 25+25, 660VA (total) - you're now looking at 35V rails, Pd = 66W, Po = 119W, my recommended rating = 2 * (66 + 119) = 740VA.

Whoops. Well, that should be close enough if you don't run it full blast -- you were shooting for a 90W/side amp anynow, right?

[See how power requirements go up with rail voltage? Not only is there more output power, there's also a lot more heat].

Caveat: I am a relative newbie to decent-amp building! But I read a lot.

Wes
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