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Headphone amp for Quad 405 amp
Headphone amp for Quad 405 amp
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Old 26th September 2021, 06:01 PM   #1
chlowden is offline chlowden
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Default Headphone amp for Quad 405 amp

Hello
Back in the 70s when the Quad 405 power amp was designed, it seems that little boxes were available that amplified the 405 speaker signal output for a headphone output. The box has a switch that either feeds the speaker signal to the headphones or continues the signal to the speakers. The 405 manual has a simple schematic that shows a provisional design.
I thought I would find such a box very easily .. but I have not. Would anyone have an idea where I can find such a thing, preferably as a kit? I am not an audiophile, so I don't need anything very sophisticated.

Many thanks
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Old 26th September 2021, 06:15 PM   #2
Mooly is offline Mooly  United Kingdom
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Headphone amp for Quad 405 amp
Welcome to diyAudio

Any device like you mention will just be a very simple resistive attenuator. Speaker position will link straight through and headphones will be via the attenuator.

It could be made to be either one or the other as selectable, or it could be headphones on all the time and speaker is switchable on and off.

If you can solder you could make one yourself.
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Old 26th September 2021, 08:21 PM   #3
chlowden is offline chlowden
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Thank you mooly for your thoughts and your welcome.
I figure it is quite simple and I have rudimentary DIY soldering ... but absolutely no electronic engineering experience. I cannot even workout how to describe this box to see schematic on the web. All the words I can think of bring up power amps.
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Old 26th September 2021, 09:42 PM   #4
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I stumbled on this box that looks like it does what I am looking for ... but not really in my price range.


Speaker Amp to Headphone Adapter Box - Listen to headphones from a speaker amp using your existing speaker cables, connect to binding posts
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Old 27th September 2021, 07:03 AM   #5
Mooly is offline Mooly  United Kingdom
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Headphone amp for Quad 405 amp
I had a look as well but couldn't find anything 100% suitable, and they were expensive.

I was searching for 'headphone'. 'speaker', 'selector', 'switch'.

Not ideal:
B Blesiya Speaker Selector Switch -Speaker Distribution Controller Box Volume Control, Supports Home Theater Stereo Amplifier Receiver System: Amazon.co.uk: Electronics & Photo

To make one you need a simple box to put it in. These can be a simple or fancy as you choose. Component suppliers have all these parts.

You need the box to be large enough to hold 4 sets of speaker sockets. Speaker in and speaker out for each channel.

You need a headphone socket.

You need a DPDT (double pole double throw) switch to switch the speaker signal.

You need two or four resistors to make the attenuator. These can be soldered direct to the headphone socket.

Electrically the circuit is simple. The negative speaker lead just loops through. The switch in the upper position links the speaker feed through unchanged. In the lower position it connects to the attenuator.

The switch (and this applies to ready built as well) must be able to handle the 100 watts output of the Quad. So a decent switch.

The resistors are chosen to suit your headphones, they cost next to nothing and its easy to work out the values. A simpler version just uses the upper resistor alone and not the two.

The main system volume controls the headphone volume, just as it does with speakers.
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Old 27th September 2021, 07:40 AM   #6
chlowden is offline chlowden
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I also found this from your key words:
DIY Audio Electronics from Zynsonix.com: Speaker Amp to Headphone Converter Box
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Old 27th September 2021, 07:49 AM   #7
Mooly is offline Mooly  United Kingdom
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Headphone amp for Quad 405 amp
That is the sort of thing but these do not seem switchable. I think you want one that allows you to switch freely between speakers and headphones which these do not seem to do.
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Old 27th September 2021, 07:55 AM   #8
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Thank you so much for your schematic. I certainly have enough to be getting on with.
Concerning the resistors , I am looking at the Sennheiser HD 25 at 70 ohms and 200mv .


Would the resistor thought from the link above, be pertinent ?
"Mills 12 watt non-inductive wirewound resistors as we don't want any induction screwing with the impedance. The Mills are well thought of for good sound quality, so it was an easy choice. I've also used the larger Kiwame 5W carbon film resistors in parallel."
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Old 27th September 2021, 08:11 AM   #9
Mooly is offline Mooly  United Kingdom
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Headphone amp for Quad 405 amp
The resistors can be altered to suit you.

I would probably begin by using a 330 ohm 2 watt for the upper one and either a 68 or 100 ohm for the lower one.

(The 330 alone would be quite acceptable without even adding the lower ones)

A metal film type would be ideal. The resistors dissipate little power in use and are only active when headphones are selected.
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Old 27th September 2021, 08:19 AM   #10
Mooly is offline Mooly  United Kingdom
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Headphone amp for Quad 405 amp
This shows what would typically happen. The amplifier is delivering 1 watt into 8 ohms (that is a typical fairly loud listening level via speakers). A level of 4 volts peak (8 volts peak to peak) corresponds to 1w/8ohm

The headphone voltage and speaker voltage are shown. By altering the resistor value/s you can get whatever level 'feels' right meaning the headphone gives a similar subjective level to the speaker without altering the volume control.
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