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Old 30th October 2007, 09:03 AM   #1
Baka is offline Baka  Montenegro
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Default Notorious question - to build or not?

I am listening music (not too loud) in a very small room (2,66m width x 3,60m length), almost near field (1,5m)

The basice question is whether is it feasible to get a good performance if I build LM3886 Gainclone for 8 Ohm/~85dB/serial crossover speakers http://www.partsexpress.com/projects...la/nebula.html (mine look way better...).

I have already red a lot about Gainclone projects but would like to confirm some issues once again.

1. Transformer: 2x25V secondaries/300VA or 2x24V secondaries/225VA (small price difference). Since I need more power than usual is that voltage sufficient for unregulated power supply? Do I need soft start for 300VA transformer?

2. I have already Noble 10k pot. Can I use it for minimalist unregulated approach only pot at the input of the chip?

3. Do I connect chassis to mains ground independently of the power supply and signal ground?

Later I would play with various options (snubbers, regulated, inverting/noninverting, with buffer or with preamp...)

I would be very grateful for any comment.

Regards to all.

Baki
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Old 30th October 2007, 03:08 PM   #2
jaycee is offline jaycee  United Kingdom
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The LM3886 chip is very good for low output powers. I've no complaints with mine

1: I would go with the 2x25V 300VA transformer if the price difference is only small.

2: Yes, should work OK... these days the low impedance shouldn't be a problem.

3: The best way to connect power ground to chassis/mains ground is via a loop breaker. Read http://sound.westhost.com/earthing.htm for more info. I would consider using the 35A rectifier as overkill.. a pair of diodes would do, as shown in http://sound.westhost.com/project04.htm - UF5404 or so would do.
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Old 31st October 2007, 11:17 AM   #3
Baka is offline Baka  Montenegro
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Quote:
Originally posted by jaycee
The LM3886 chip is very good for low output powers. I've no complaints with mine
What is the sensitivity of your speakers?

Are 8 Ohm/~85dB/with serial crossover speakers tough load to achieve a moderate/reasonable loudness but with good control? What is your experience?


Quote:
1: I would go with the 2x25V 300VA transformer if the price difference is only small.
Should I need a soft start circuit in this case?

Thanks jaycee
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Old 31st October 2007, 11:40 AM   #4
AndrewT is online now AndrewT  Scotland
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Hi,
300VA toroid will start without blowing the 20A distribution board circuit breaker.
You may find that the usual rule of fuse current capacity = 3*VA/Vac will give reliable starting without blowing the fuse too often. But that is a big fuse (T4A).
If you want to go down the road of close fuse rating then soft start is essential even for a little toroid.
About 50r to 75r in series with the primary winding will probably allow a T2A to be used and maybe even smaller (T1.6A). Arrange a relay bypass that activates about 300mS to 500mS after switch on.
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Old 31st October 2007, 11:46 AM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
50W into 85db/W speakers will go quite loud in a small room, with mid field listening position.

Allowing 20db average to peak means using about half a watt.
This will give about 83db from the speaker @ 1m. The 1.5m listening volume will be about 79db average level and giving peak potential of 99db.
That would sound pretty loud for most listening. I suspect you could end up using 100mW most of the time.
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Old 31st October 2007, 02:26 PM   #6
Baka is offline Baka  Montenegro
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Many thanks Andrew.

Well, let the games begin!
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Old 1st November 2007, 03:31 PM   #7
jaycee is offline jaycee  United Kingdom
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Quote:
Originally posted by Baka
What is the sensitivity of your speakers?

Are 8 Ohm/~85dB/with serial crossover speakers tough load to achieve a moderate/reasonable loudness but with good control? What is your experience?
My speakers are Eltax Liberty 5+, which are 87dB sensitivity. With a 160VA transformer powering 2 channels, using 4x4700uF capacitance total (9400uF per rail) I can go pretty damn loud without running out of steam

My circuit is not strictly Gainclone... but it is using LM3886. My circuit has small decoupling caps (470uF+100nf) on the PCB near to the LM3886, and then a conventional capacitor bank supplying both channels.

A picture can be seen here. The transformer has since been replaced with a 160VA toroid.

Quote:
Originally posted by Baka
Should I need a soft start circuit in this case?
For 300VA it would be wise to do that. There's a few circuits around. If you're not comfortable with doing that (it is after all circuitry that has to switch high voltage) then 160-200VA should be OK with a regular time-delay fuse.
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Old 2nd November 2007, 07:12 AM   #8
Baka is offline Baka  Montenegro
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Many thanks, jaycee.

Will start ordering.
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