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Old 27th November 2002, 01:54 PM   #11
halojoy is offline halojoy  Sweden
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5 times gain
is a good choice for HP-amp
that is going to work up to 600 ohms
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Old 27th November 2002, 02:14 PM   #12
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Default Amp design

Hi Halojoy,
I wanted the circuit to be useful as a preamp and a headphone amp for a range of impedances. I have a range of phones from 32 to 600 ohms.
I will have this design built by this weekend. I have some decent measurement equipment and will put up the results as soon as I complete the tests.
Should be interesting.
I wanted to simulate the design with the 6DJ8 or 6922 but I do not have the Spice data that is useable with Circuitmaker .
Cheers.
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Old 27th November 2002, 04:29 PM   #13
halojoy is offline halojoy  Sweden
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Default Re: Amp design

Quote:
Originally posted by ashok
Hi Halojoy,
I wanted the circuit to be useful as a preamp and a headphone amp for a range of impedances. I have a range of phones from 32 to 600 ohms.
I will have this design built by this weekend. I have some decent measurement equipment and will put up the results as soon as I complete the tests.
Should be interesting.
I wanted to simulate the design with the 6DJ8 or 6922 but I do not have the Spice data that is useable with Circuitmaker .
Cheers.
Yes, that is a the good thing with headphone amps.
They can work as lineamps, with great drive capacity downto the "magical" 600 ohms. 1mW in 600 ohms= 0.77459 Vrms, some kinda old standard
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Old 27th November 2002, 04:35 PM   #14
dhaen is offline dhaen  Europe
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Quote:
1mW in 600 ohms= 0.77459 Vrms, some kinda old standard
=0dBm
Still in use, but more often used is dBu
This is the unterminated equivalent.
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Old 30th November 2002, 01:35 AM   #15
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Have you looked at the headphone amp in this months Valve
page over on the Bottlehead web page. It sure looks cute
and easy.

Regards,
bob12345678
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Old 30th November 2002, 03:15 PM   #16
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Default Circuit update

Hi Bob,
I had a look at the circuit you refered to. Yes it looks fine and it uses batteries keeping the 'ripple' problems away. Unfortunately I cannot find those tubes here and lantern batteries are not so common here.

I looked the circuit I put up earlier and found that like many low feedback design's this has very poor ripple rejection. I wanted at least 80db ripple to signal ratio ( at 1mW into 300 ohms). So I had to design a low ripple regulated power supply and this was not enough. So I modified the circuit some more and the result is shown in the diagram below.
The anode load has been split into two resistors with a capacitor going to ground. This cuts some of the supply induced ripple and the capacitor across across the cathode resistor which increases the stage gain a bit and cuts ripple by increasing the apparent negative feedback.
I am building the circuit and will put up the results soon.
Cheers.

Note V4 is the 'ripple' supply used in the simulation and is not normally part of the circuit. The input signal has been set to 'o' volts. In Circuitmaker this simulation gave a s/n of about 84db with respect to 0.5V rms output.
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File Type: gif headphone-pre amp-e.gif (6.7 KB, 809 views)
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Old 30th November 2002, 04:42 PM   #17
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Default Regulated supply

I simulated this supply circuit and its performance seems to be good enough.
I anyone has any suggestions please let me know before I build this circuit.
The passive component values may change a bit when it is finally done.
Cheers.
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File Type: gif headphone supply.gif (3.9 KB, 439 views)
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Old 30th November 2002, 04:52 PM   #18
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Default Power supply update

Sorry. The previous post has the wrong supply circuit.
The actual one is given below.
Cheers.
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File Type: gif phones pow sup-a.gif (4.4 KB, 701 views)
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Old 3rd December 2002, 04:49 PM   #19
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Default Built and tested

Hi, I finally completed the amp after several problems and blown MOSFET's.
I am attaching the frequency response measurement. The amp is essentially flat without a load. With a fluctuating load like a headphone the response is withing +/- 0.3 db from 20 to 20 Khz.
The sound is very good over headphones. The transients are fabulous and bass goes deep with a Sennheiser HD-580 headphone.
More details later along with a picture.
Cheers.
I tried the Sheffield Drum CD and Linda Ronstadt - Winter Light. Have not had time to try other disc's.
Cheers.
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File Type: gif frequency response.gif (15.3 KB, 365 views)
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Old 3rd December 2002, 04:57 PM   #20
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Default Response graph.

The response graph has three curves. The one at 0db is the amp into an open load. The next lower one is into a 300 ohm HD-580 headphone and the lowest one is the response into a Sennheiser HD-475 60 ohm headphone.
The distortion curve attached below is into a 150 ohm load. The kink around 2 volts is probably the region where the amp goes from class A into class AB operation. Note that most of the signal is below this ( less than 1.5 volts peak ) when listening to music on headphones. Into high impedances of 5K ohms or higher , the amp is essentially in the class A mode. Distortion is not very low as the feedback is quite low. I will test the same circuit with the 6922 tube and post the performance.
Note that the noise floor is quite low around -80db and is quite inaudible on the headphones.
Cheers.
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File Type: gif distortion.gif (12.6 KB, 355 views)
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