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Commercial Gainclone kit- building instructions

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M

MrOfficer

Member
Joined 2011
AndrewT said:
despite quoting the text of my advice you are again asking for my opinion, so I'll repeat my opinion
AND keep the loop areas small. That means no single wires trailing randomly across the enclosure.

Is that an audio sector PCB?
It states "technofreek.com"
I think you are in the wrong section.
Click to expand...

Rectifier board/parts are from Audiosector, AMP boards are not.

Transformer is 18 0 18 and I have 2 X 10,000 uf/50V with snubber parts on rectifier (single bridge/single transformer)

This is what I did, added a jumper between PG+ & PG- on rectifier and a jumper between output grounds on Amp pcb (speaker output ground(s) are also taken from these points) , both these jumpers are connected with a wire. The center tap from transformer is connected to the middle of this wire.

AC mains usually hover around 255 Volts, transformer secondary shows 20 0 20 and when I measure from center tap on transformer and V+ & V- on amp boards I get +/- 28 VDC

Without a volume pot and any input source I get 77 mV on one board and 125 mV on other.

Does this all sound OK ?
 
P

Paulp1960

Member
Joined 2016
LM3875 kit input sensitivity

Hi I wonder if somebody could advise me. I already have one LM3875 kit from Peter but not built yet.

I really want to build a 4 channel amp for active home-made speakers. I was planning on using a MiniDSP 2x4 for the crossover and eq and volume control.

The original MiniDSP 2x4 has a maximum output of 0.9V rms. The MiniDSP 2x4 HD has a 2.0V rms output.

Ideally I would like the amps to be running at maximum power when the MiniDSP outputs a full signal.

Any ideas how close a match this would be with either MiniDSP or any recommendations for changing the gain on the chip amp to suit.

Ideally I would go for the 2.0v MiniDSP 2x4 HD.
 
M

MartyBoy

Member
Joined 2016
viagrasansordonnancefr.com/
Hi Peter,

Finished building My amp with your premium kit, wich I have been enjoying a lot since the beginning of 2017. Fantastic setup!

Thanks everyone also for the deeply valued replies and comments.

While listening with my headphone last week, I noticed a very low volume "popcorn", scratchy sound coming from left channel only. Been testing every combination and it's not the speakers. It is not volume dependant either. I only notice it when volume is very low, between tracks.

Checked rails voltages (31V and -31V), dc offset (5mV to 32mV).
R1 is a piece of wire


Everything seems to play normally. But I am just a bit worried that it hides something worse over time. What could it be????
 
A

AndrewT

R.I.P.
Joined 2004
is you amp inside a metallic enclosure?

If so then you can add RF attenuation where the cables enter the enclosure.
I generally use 47pF from Signal Hot/Flow to Signal Cold/Return on the back of the RCA/XLR socket.
And 1nF on the speaker terminals from both Hot and Cold to Chassis.
And a mains IEC socket with an integrated canned filter. Select the lowest current rating to get the highest RF attenuation.
 
M

MartyBoy

Member
Joined 2016
viagrasansordonnancefr.com/
AndrewT said:
attach the two resistors in series. That gives you 101r
Attach the 100r to the Hot/Flow output.
Attach the 1r to the Cold/Return output.
The headphone Hot/Flow output comes from the junction of the 100r & 1r
The headphone Cold/Return output comes from the amplifier Cold/Return.

This way the headphones see a source impedance of a tiny bit less than 1r
and the amplifier sees a load a bit higher than 100r

If the 100W amplifier were turned up to just reach clipping the 100r will dissipate less than P=28.2V²/100r = 8W
Use at least 16W of resistor for this arrangement to get 40dB of attenuation from a 100W amplifier.

Use your own values to determine your attenuation and impedances and dissipations.
Click to expand...

mmm ok .

My headphone jack has 1x left, 1x right, and the ground. Both my output ground are connected togheter at the jack. How do implement the 1r resistor? I'm confused...

So far, I've just connected 1x 100r to both left and right channel hot signal.
 
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U

udailey

Member
Joined 2008
picasaweb.google.com/udailey/MyRef#
So you nearly short out each headphone speaker with a 1R resistor? Best case scenario is that you have 32ohm headphones. Worst case is 600ohm headphones. So 28V out of the amp gives you around a quarter of a volt to drive the headphones? This won't provide enough volume.
If I wanted to drive headphones with a chip amp I would change the gain of the amp to 10 for lm3886 or lm3875. Then I would add a permanent attenuation circuit on the input. If I were super cautious I might put a 100R resistor on the output of the amp and it would be a low wattage resistor because headphones inherently contribute to the total resistance seen by the amp resulting in very low power output.
Unless you have some way to mute the amp on power up and down I would not leave my headphones plugged in during turn on and off.
 
U

udailey

Member
Joined 2008
picasaweb.google.com/udailey/MyRef#
Maybe something like this schematic. This schematic ignores capacitors and power supply. Just giving an idea for attenuation and gain as well as current limiting the output.
If you have a pop on turn on then it could blow the headphones if they are plugged in when you turn it on. If you turn it off with headphones plugged in you could have a problem with too much DC getting to the phones as a lot of times the amp can have large DC offset while turning off.
 

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A

AndrewT

R.I.P.
Joined 2004
udailey said:
So you nearly short out each headphone speaker with a 1R resistor? Best case scenario is that you have 32ohm headphones. Worst case is 600ohm headphones. So 28V out of the amp gives you around a quarter of a volt to drive the headphones? This won't provide enough volume.
If I wanted to drive headphones with a chip amp I would change the gain of the amp to 10 for lm3886 or lm3875. Then I would add a permanent attenuation circuit on the input. If I were super cautious I might put a 100R resistor on the output of the amp and it would be a low wattage resistor because headphones inherently contribute to the total resistance seen by the amp resulting in very low power output.
Unless you have some way to mute the amp on power up and down I would not leave my headphones plugged in during turn on and off.
Click to expand...
if 100r+1r gives too much attenuation (-40dB) then change to 100r+3r3 to give -30dB
Don't go and change the stability margins of the chipamp by reducing it's gain.
 
sthcoaster

sthcoaster

Member
Joined 2016
Paid Member
No. Just in place of the 2x 1500uf Panasonic FCs usually recommended on the amp board. Using same value of Mundorf Mlytic AG. I have them so will give them a try anyway - just curious about other's experiences of using them. Also going to try 10k Supertech caps + snubber on the rectifier board (but not at the same time :))
 
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