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Old 31st May 2010, 03:04 AM   #1
Lavcat is offline Lavcat  United States
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Default FET 10/HL to drive headphones?

For about twenty years I happily used my FET 10/HL with Denon monoblocks. About two years ago one of the Denons died in an unfortunate trichobezoar incident. The other Denon had been developing hum for a few years, and this left me nothing for the FET 10 to drive. (And no functional music system either.) Replacing the monoblocks is financially not an option, but I have since become a headphone addict, and am thinking of using the FET 10 to drive headphones.

I wanted to ask if I am at risk of damaging the FET 10 with a load in the 32-300 ohm range? Should I consider adding circuitry to protect the amp, or to protect the 'phones for that matter?

From what I've read here and from the simplified FET 10 schematic that Nelson posted a few years ago, it looks like the emitter follower should work well as long as the output devices can supply the current. Does anyone know what the output transistors are? The part numbers of the output devices were not marked on what Nelson posted. I've never opened the FET 10.

By any chance has anyone tried headphones with the FET 10, or a similar design?

Thanks.
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Old 31st May 2010, 03:39 AM   #2
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Default No !

mpsa42/92 can't drive any too low impedance (32 Ohms) but it can drive 300 & 600 with very low level of sound.

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Anadigit
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Old 4th June 2010, 02:52 AM   #3
Lavcat is offline Lavcat  United States
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Quote:
Originally Posted by ANADIGIT View Post
mpsa42/92 can't drive any too low impedance (32 Ohms) but it can drive 300 & 600 with very low level of sound.

Thanks
Anadigit
Thanks for the response.

Today I received an RCA to 1/4 inch adapter, so I could test the FET 10/HL as a headphone amp. There is plenty of volume, using a Sony SACD-1 as source. Using 300 ohm headphones maximum tolerable listening level was with an attenuator setting of 12 o'clock, for orchestral music. With 32 ohms (Denon D7000) the maximum tolerable listening level was about 1 o'clock. I don't know why one would want more volume? For most non-orchestral albums my normal listening level would be about 9 or 10 o'clock on the attenuator.

I know the damping factor must be rather low in this application, but sound quality is excellent. However it will take more than a couple of hours of critical listening to be sure of the assessment.
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Old 4th June 2010, 05:46 AM   #4
EUVL is offline EUVL  Europe
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Add a DAO follower (unity gain) and you can drive anything down to 25 ohms.

Some other Source Follower COnfigurations


Patrick

Last edited by EUVL; 4th June 2010 at 05:50 AM.
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Old 7th June 2010, 04:47 AM   #5
Lavcat is offline Lavcat  United States
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Quote:
Originally Posted by EUVL View Post
Add a DAO follower (unity gain) and you can drive anything down to 25 ohms.

Some other Source Follower COnfigurations


Patrick

Nothing like making (at least) two errors in my first thread. (The other being the model of the player I was speaking of is SCD-1, not SACD-1.)

Last fall when I read about the Lovoltech parts I thought they would make a nice headphone amp, but I didn't do anything more than think about it for a couple weeks. The problem of DC offset also bothered me. That was before I had the idea to try the FET 10 with headphones.

I'm reasonably sure the FET 10, like most preamps, was not designed to drive headphones, but what would make it unsuitable? In principle I don't like the idea of adding additional active stages if fewer devices work well, and adding a buffer would be driving one buffer with another.

A quick calculation: the D7000's have a specified sensitivity of 108 dB SPL/mW. Let's assume (to make the calculation easier) we want 128 dB SPL. That should be 0.1 W, or 63 mA at 1.58 V. Instead of "First Watt", maybe we should be thinking "First Milliwatt".
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Old 7th June 2010, 05:11 AM   #6
EUVL is offline EUVL  Europe
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A DAO follower will take at most 2 hours to make by P2P wiring (just follow PCB layout roughly).
You can then test it with a lab supply, and you have the answer for yourself.

Or ask Steen (or one of the others who built one) by PM.




Patrick
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