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Old 1st January 2017, 03:37 PM   #2171
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Quote:
Originally Posted by AndrewT View Post
Use an attenuated output for the headphones.
A 100r:1r will cut the volume by 40dB and not blow up your headphones if you had left the vol pot at max. This gives the headphones a source impedance of ~1ohm. If you phones need a higher source impedance, you can increase the resistor values
How does one implement such a resistor? In parallel between the 2 channels at the headphone jack?
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Old 2nd January 2017, 08:07 AM   #2172
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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=28.2V/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.
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Old 2nd January 2017, 02:43 PM   #2173
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Default 1875 or 3875?

Quote:
Originally Posted by Peter Daniel View Post
While the projects described so far were using printed boards, I would like to present now an elegant way to build an amp circuit without PCBs, using point to point wiring technique.

It so happens that I recently received an order for LM1875 amp, which I never tried before, so there was not other way as going p2p.

The amp circuit as presented in National's datasheet:
Hi Peter,
As you had the chance to listen to lm1875 and lm3875 in the same configuration and setup, could you tell me the main differences in sound between them? the output power is not a big issue, since my speakers are 96 dB efficient. Sorry for my poor English. Thanks in advance!
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Old 14th January 2017, 01:49 PM   #2174
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The important aspect about sharing a single transformer and common rectifiers per two amp channels is power star ground.

While you could simply run PG+ and PG- (ground) wires from rectifier board to each amp board, in some setups hum problems were reported and presently I always use star (power) ground in all my amps.

I simply achieve it by connecting output grounds (OG, on the opposite side of speaker wire connection) with a 14ga solid core copper wire.

The center of that wire is my power star ground and both PG+ and PG- grounds from rectifier board connect here. In the picture below the connections are so short that I used single runs of wire, normally I use two wires for both PG+ and PG-.

Please note that power ground is also directly connected to the chassis.

I specifically refer to it as power star ground, as signal wires from RCAs are not connect directly here. They connect through separate traces on PCB to OG (output ground) pads.

Should the center tap of transformer secondary also connect here ?
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Old 14th January 2017, 04:05 PM   #2175
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The centre tap is part of the capacitor charging circuit.
The centre tap must go to the smoothing capacitors.
No where else.
And keep the loop areas very small. That means use a twisted triplet from transformer to rectifier and from rectifier to smoothing capacitors and from smoothing capacitors to amplifier.
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Old 14th January 2017, 04:17 PM   #2176
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Quote:
Originally Posted by MrOfficer View Post
Should the center tap of transformer secondary also connect here ?
If you are using my rectifier boards, which are part of the amp kit discussed here, then center tap of transformer secondary does not connect there (wire bridge between OG pads on amp boards).

My rectifier boards are actually designed to work with dual secondaries transformers, not center tapped units, however, there is a mod for that, when center tap transformer can also be used: Center tap torroid wiring to peter daniels kit???

In either case, secondary wires from transofrmer connect to rectifier boards and not to the wire bridge on amp boards.
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Old 14th January 2017, 04:22 PM   #2177
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Quote:
Originally Posted by peppennino View Post
Hi Peter,
As you had the chance to listen to lm1875 and lm3875 in the same configuration and setup, could you tell me the main differences in sound between them? the output power is not a big issue, since my speakers are 96 dB efficient. Sorry for my poor English. Thanks in advance!
I didn't really have much chance to listen extensively to this amp as I rushed to sent it to the customer who requested that chip configuration.

To me, it's a bit too low in power and as such not practical. People say LM1875 sounds even smoother, but I can't really confirm it.
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Old 14th January 2017, 06:18 PM   #2178
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Thanks, Peter.
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Old 15th January 2017, 04:47 AM   #2179
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Quote:
Originally Posted by AndrewT View Post
The centre tap is part of the capacitor charging circuit.
The centre tap must go to the smoothing capacitors.
No where else.
And keep the loop areas very small. That means use a twisted triplet from transformer to rectifier and from rectifier to smoothing capacitors and from smoothing capacitors to amplifier.
Quote:
Originally Posted by Peter Daniel View Post
If you are using my rectifier boards, which are part of the amp kit discussed here, then center tap of transformer secondary does not connect there (wire bridge between OG pads on amp boards).

My rectifier boards are actually designed to work with dual secondaries transformers, not center tapped units, however, there is a mod for that, when center tap transformer can also be used: Center tap torroid wiring to peter daniels kit???

In either case, secondary wires from transofrmer connect to rectifier boards and not to the wire bridge on amp boards.
Conx - TBD.jpg

Conx 1.jpg

AMP pcb.jpg

Is this is how all the wiring should be done ?
Please excuse shoddy MS Paint job

Last edited by MrOfficer; 15th January 2017 at 05:14 AM.
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Old 15th January 2017, 09:18 AM   #2180
AndrewT is offline AndrewT  Scotland
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despite quoting the text of my advice you are again asking for my opinion, so I'll repeat my opinion
Quote:
use a twisted triplet from transformer to rectifier and from rectifier to smoothing capacitors and from smoothing capacitors to amplifier.
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.
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Last edited by AndrewT; 15th January 2017 at 09:22 AM.
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