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Messages - RDV

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16
Amplifier Discussion / Re: LM3886 Power Supply question.
« on: July 14, 2006, 06:56:06 AM »
If you have questions, ask them.

RDV

17
Amplifier Discussion / Re: PT for LM3886 project?
« on: July 13, 2006, 02:55:48 PM »

18
Amplifier Discussion / Re: LM3886 Power Supply question.
« on: July 13, 2006, 08:15:40 AM »
AC1 should be(the wires from) one secondary, and AC2 the other, there is no polarity.

RDV

19
Preamps and Effects / Re: Driving an SS Power Amp?
« on: July 11, 2006, 09:13:27 PM »
Steve:

I would add a DC blocking cap in front of the LM3886 of 1µF or more just to be safe.

RDV

20
Amplifier Discussion / Re: How to measure a transformer?
« on: July 10, 2006, 02:14:33 PM »
I'd buy a transformer myself.

RDV

21
Amplifier Discussion / Re: Crate CR-212.....HUMMMMM
« on: July 09, 2006, 11:01:26 PM »
That D.C offset is pretty high, but not so high as to stop the amp from working or anything.

I'm wondering now if this is just the way this amp sounds. I can only suggest larger PS caps like 10,000µF or even higher but that still may not do it or even be worth the cost on such an old amp.

Sometimes designers shoot themselves in the foot by crowding things together and causing proximity effect hums and buzzes. Of course I'm guessing here cause we don't have a schematic or layout to go by here.

Hum usually means DC where there isn't supposed to be any. A Zobel network on the output might stop some oscillations. It will require a 2.7 ohm 2 watt resistor and a .1µF cap.

Like this.

HTH

RDV

22
Amplifier Discussion / Re: Crate CR-212.....HUMMMMM
« on: July 09, 2006, 08:25:00 PM »
Disconnect the speakers and turn on the amp and measure the DC offset by measuring the + & - speaker terminals and post the voltage reading. If it is high, then you likely have a bad transistor or a short somewhere.

I used to have a Crate G-60 which is pretty much the same amp. It used to hum like crazy and I could hit it on the top of the amp & it would stop! Eventually it started switching channels all by itself and I took it apart. Probably could've fixed it now.

RDV

23
Amplifier Discussion / Re: LM4766TF/LM4752T
« on: July 09, 2006, 05:15:31 PM »
I've not heard of one specifically but there are many based on the LM4780 which is very similar. Maybe even pin for pin, I haven't checked. It's a pity the LM4780 isn't available in the TF isolated version. I'm in the middle of a stereo paralled 100 watt per channel LM4780 build. Waiting on parts.

RDV

24
Amplifier Discussion / Re: Crate CR-212.....HUMMMMM
« on: July 09, 2006, 04:03:32 PM »
Should the preamp out and the poweramp in jacks be isolated as well?
Were they originally isolated? If so yes, if not you could try it and see.

RDV

25
Amplifier Discussion / Re: How to measure a transformer?
« on: July 09, 2006, 03:51:19 PM »
Side 1
Pin1 and 2 - 8,7 ohm = primary =Pin 1 = hot?
pin1 and 3 - 1.2 ohm = Pin 2 = common?
pin2 and 3 - 9.5 ohm = primary = Pin 3 =  hot?

side 2
pin1 and 2 - 0.9 ohm = secondary =Pin 1 = hot?
pin1 and 3 - 1.5 ohm = secondary = Pin 2 = common?
pin2 and 3 - 1 ohm    = secondary = Pin 3 = hot?

My guess would be side 1 being primary, with side 2 being CT secondary. When testing this, you really need to be careful

When taking apart the Pioneer amp it would have been useful to have noted where the mains voltage entered the transformer.

Is the side with the dot side 1?

I'd buy a transformer myself.

RDV

26
Amplifier Discussion / Re: How to measure a transformer?
« on: July 09, 2006, 12:53:20 PM »
The red dot may just be polarity marking also.

Here's one way to do it if you have a decent DMM. Set the DMM to measure resistance and on the lowest range available cause you'll be measuring fractions of an ohm.

You should see:

3.3 ohms +/-  between the primary wires.

.8 ohms +/-  between the secondary pairs.


I've tested a few this way and it seems to work pretty consistantly.

HTH

RDV

27
Amplifier Discussion / Re: Crate CR-212.....HUMMMMM
« on: July 09, 2006, 03:21:42 AM »
The hum is probably not coming from the poweramp as you get it from the line out as well.

If you access the power via the footswitch jack is the hum there then? If not we can narrow the problem to the preamp & or preamp power supply. Without a schem it's hard for me to know what's going on there. If the preamp gets it's power from the main supply, it either does it with regulators, or with zener diodes. I doubt it has a seperate transformer or anything.

Try isolating the jacks in front cause that could be a ground loop source and a relatively cheap solution if so. In the Crates I've owned they were plastic jacks which would be isolated so that may be it.

If that doesn't do it then we'll need to examine how the preamp is getting it's power and see if it's getting too much or too little or if you getting ripple in there causing oscillation at a low frequency.

RDV

28
Amplifier Discussion / Re: Crate CR-212.....HUMMMMM
« on: July 09, 2006, 12:27:12 AM »
First, welcome, and we'll help if we can.

I've got some questions for you.

Did you own this amp before it hummed? Or do you just get it, and it was humming when you got it? If there is a headphone jack or line out, does it hum from those outputs. If it does not hum from those outputs then we can narrow it down to the power amp.

You say you replaced caps, what portion of the amp were these located in? Power Supply, Preamp, or power amp sections?

This is quite an old amp, you could have a bad transformer or power transistor quite easily.

RDV

29
Preamps and Effects / Re: Hiwatt-based preamp
« on: July 07, 2006, 09:13:18 PM »
Wow, I yak about me a lot on line don't I! :duh

I had a Hiwatt that never worked right till I had it retubed and biased and then found out that no one wanted to hear me that loudly so I sold it to a heavy metal bass player.

I may try out the circuit myself this weekend. I'll let you know.

RDV

30
Amplifier Discussion / Re: PS caps..?
« on: July 07, 2006, 09:08:21 PM »
You could try it, and I'm sure it would work, but most every PS you see for this purpose has at least 10,000µF per rail(sometimes more). An exception seems to be here at http://www.audiosector.com/ . You can always change them out if it hums. The Xicons I use are only $3.67 each. The Panasonic ones are about $5.00 but hard to source except at Digi-key.

HTH

RDV

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