Is 56w needed?

[TheDriver41]

I was set on a Avel 330VA 25V+25v transformer for my two channel gainclone project. But after research horns I am second guessing the decision. The speakers I am looking are the Madisound Fostex BK-16 Folded Horn kit.

http://www.madisound.com/BK16.html

They have a sensitivity of 95dB.

Does maxing out the LM3875 hurt sound quality?

Would it sound better with the 56w output or a lower output?

Also on a side note how much of a difference would I see by making monoblocks?

Thanks,
Matt

[planet10]

Personally, i'd stay away from the max. a trafo in the 18-22V range is probably more appropriate.

dave

[GregGC]

Originally posted by TheDriver41
I was set on a Avel 330VA 25V+25v transformer for my two channel gainclone project. But after research horns I am second guessing the decision. The speakers I am looking are the Madisound Fostex BK-16 Folded Horn kit.

http://www.madisound.com/BK16.html

They have a sensitivity of 95dB.

Does maxing out the LM3875 hurt sound quality?

Would it sound better with the 56w output or a lower output?

Also on a side note how much of a difference would I see by making monoblocks?

Thanks,
Matt

95 db speakers will require relatively small amount of power from the GC if you like normal listening levels. So whether the amp is capable of giving 50 or 30 w will make a little dif. To size the PS properly you'd want to know the Speaker impedance. For 4 ohms or less go with lower voltage (18V RMS transformer) and for 6 and up 22-25 VRMS. With high efficiency speakers you can try setting the gain of the GC to somewhere around 15-20 and see how it'll sound to you compared to gain of 25-30.

Good luck.
/Greg

[TheDriver41]

Originally posted by GregGC


95 db speakers will require relatively small amount of power from the GC if you like normal listening levels. So whether the amp is capable of giving 50 or 30 w will make a little dif. To size the PS properly you'd want to know the Speaker impedance. For 4 ohms or less go with lower voltage (18V RMS transformer) and for 6 and up 22-25 VRMS. With high efficiency speakers you can try setting the gain of the GC to somewhere around 15-20 and see how it'll sound to you compared to gain of 25-30.

Good luck.
/Greg

I will be using 8 ohm high efficiency speakers, 95dB. I have been looking at the data sheet for the LM3587 trying to understand why 4 ohm speakers want 18v and 8 ohm want 22v. Can you help me understand the why?

Correct me if I'm wrong but if I lower the gain that will give me more bandwidth but less output. What exactly is bandwidth? And if all I want is 5w couldn't I lower the gain to get the most bandwidth target 5w? Could I put a variable in the gain loop and just play with it?

[GregGC]

The reason you want less voltage if you have 4 and less ohms load is because the chip starts to dissipate more power. By lowering the PS voltage the power dissipation of the chip drops also and the protection circuit won't get activated. So 18 V is recommended for 4 ohms speakers and if you want to get max power from the chip. You can still use 22V if your max output power is far from the max as it is in your case.

I find in a commercial amps that have a setting for the load a lot of people set the 4/8 ohm load switch to 4 ohms when they drive 8 ohms speakers thinking that that's a better setting, but all they get is reduced output power because all the switch does is drops the PS voltage for 4 ohm loads.

You are correct that the frequency bandwidth (min frequency minus max frequency) is wider at lower gain, though at gain of 30 you have FH3dB=200kHz which is very good any way, so I don't think the bandwidth is something you can improve much on. It's already quite good as it is. If all you need is 5W you can use a trim pot to change the gain but I don't recommend it. Just set the gain to 15 or 20 and see how it'll sound. You shouldn't set the gain below 10 (20 dB) because the amp may become unstable. So you don't have too much to play with the gain anyway.

Looking forward to hearing about the results of the exercise.

/Greg

[sam9]

The choice of transformer voltages will determine the peaks that can be amplified without clipping. If will not affect the average or low passages in the source material unless you listen space is so large and you listen so loudly that your choice of IC is generally inadequate. A general rule of thumb is that you need to double wattage to get a noticable increase in loudness.

However, if the capabilities of the IC are OK for nearly all of the source material the higher voltage may be helpful. Although distortion will shoot up on the peaks, that will still probably sound better than the clipping that will occur with lower rails.

[KCHANG]

Hi Matt,

I have been using a Gainclone to drive my Fostex FX200 drivers on open baffles. I have several power transformers that after ractification give me from +-18V to +-35VDC. Even using the one with the lowest voltages (it's a cheap 25.2VAC CT, 2A transformer from Radio Shack, BTW), the gainclone can drive the FX200's to a volume more than loud enough for me in my about 1400SQFT basement. The efficiency of the FX200 is only 92dB. I also got enough volume from my Jordan JX92S in sealed boxes driven by a Gainclone, even though the efficiency of the Jordan driver is not even 90dB.

Kurt

[TheDriver41]

Thanks guys for all the great input. From everything I have learned in this thread I have refined my tranny selection to three.

1) Plitron 067014011, 18V @ 6.25 amps, 225VA, 7% regulation, $30.00
http://www.leedselect.com/parts-transformers.html

2) Plitron 047015201, 22V @ 2.73 amps, 120VA, 11% regulation, $54.39
http://www.plitron.com/pages/order.htm

3) Plitron 077015201, 22V @ 6.82 amps, 300VA, 6% regulation, $62.95
http://www.plitron.com/pages/order.htm

I like the price on the first option. It's so cheap I can make monoblocks for the same price as the other two. But I am afraid of clipping with the lower voltage. That's why the other two are 22V. I would use option two for monoblocks and option three for a 2-channel operation. I like the idea of monoblocks, but don't know if it’s worth it if I go 22v, $110 vs $63. What does % of regulation mean? And what do you guys think about the options?

Thanks,
Matt

[mateo88]

I would say go for the first option. I've built gainclones with 25v (35v rectified) and 18v (24v rectified?? - never measured) secondaries. The 18v gets louder than my taste with 89 db efficient speakers, and I haven't had the 25v up to full volume before.

[planet10]

Originally posted by TheDriver41
1) Plitron 067014011, 18V @ 6.25 amps, 225VA, 7% regulation, $30.00
http://www.leedselect.com/parts-transformers.html

That is a VERY good price for those. I've got a number of these and i save them for my highest quality amps.

Since no speaker is anywhere near a resistor in terms of what the load looks like, this is also what i think is the best voltage for a 3875... you get the most linear peformance into ugly loads.

using these for monoblocs is the "supercharger + nitros" level (you need to read Allen Wright's Tube Pre-amp Cookbook to get that one). 1 serves fine for a stereo amp.

dave