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Old 8th February 2013, 12:11 PM   #321
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Quote:
Originally Posted by merlin el mago View Post
regal wrote: "I know this is not good for analog guys, but if you have the bucks and space for analog you probably aren't a headphone guy anyway."

I don't understand your reasoning.
It means:

If your house has big rooms....... big enough to have room for LP players, and 1000s of LPs, you probably have space for loudspeakers, and don't need headphones.
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Old 8th February 2013, 12:13 PM   #322
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Thanks Rod, I changed the answer, in place or understand I wrote "don't agree".

I understood headphones = small space, loudspeakers = large space.
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Old 8th February 2013, 12:14 PM   #323
regal is offline regal  United States
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Quote:
Originally Posted by merlin el mago View Post
regal wrote: "I know this is not good for analog guys, but if you have the bucks and space for analog you probably aren't a headphone guy anyway."

I don't agree your reasoning.
I'm just being a little sarcasitic and jealous of analog owners is all
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Old 8th February 2013, 12:21 PM   #324
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The sound system that initiated me in the audio world in 1974 was an integrated amplifier Copland with MM valves, loudspeakers 3 way passive Dynaudio and TT Linn Sondek LP12 with Linn MM K9 cartridge. After a couple of years I discovered Stax but never had enough money to buy because I wanted the combo: headphone amp + headphones.
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Old 8th February 2013, 01:05 PM   #325
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We have to consider ultrasonic filtering. I know many designs do not trouble to do it.

A single pole can be added by parallel cap across the output resistor.

Then again, the whole analogue subsytem, from DAC out to headphone socket - is open-loop.

Feedback can present problems if the forward path of an amplifier is not able to keep pace with the spectrum of quantisation and other noise. With open-loop design, and discrete components, there is far less reason to suspect trouble - maybe!

What DACs are we using? ESS9018? PCM170x ?

Volume control is another consideration - it's a shame to break up this single-current-path stage to accommodate a pot. Are the DAC-internal digital volume controls good enough yet?
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Old 8th February 2013, 01:23 PM   #326
regal is offline regal  United States
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erno borbely told me the PCM1704 is the best DAC chip ever made, so I took his word, been working with it for years never found anything better. It doesn't allow digital volume control so a potentiometer or autoformer is unfortunately a requirement but its filtering requirements are much less since it doesn't operate in the hf's that sigma delta chips do, generally a CLC filter is enough.
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Old 8th February 2013, 01:31 PM   #327
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Quote:
Originally Posted by Rod Coleman View Post
We have to consider ultrasonic filtering. I know many designs do not trouble to do it.

A single pole can be added by parallel cap across the output resistor.

Then again, the whole analogue subsytem, from DAC out to headphone socket - is open-loop.

Feedback can present problems if the forward path of an amplifier is not able to keep pace with the spectrum of quantisation and other noise. With open-loop design, and discrete components, there is far less reason to suspect trouble - maybe!

What DACs are we using? ESS9018? PCM170x ?

Volume control is another consideration - it's a shame to break up this single-current-path stage to accommodate a pot. Are the DAC-internal digital volume controls good enough yet?
I own both.

Quote:
Originally Posted by regal View Post
erno borbely told me the PCM1704 is the best DAC chip ever made, so I took his word, been working with it for years never found anything better. It doesn't allow digital volume control so a potentiometer or autoformer is unfortunately a requirement but its filtering requirements are much less since it doesn't operate in the hf's that sigma delta chips do, generally a CLC filter is enough.
Erno now owns also ESS9018
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Old 8th February 2013, 01:47 PM   #328
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Quote:
Originally Posted by Rod Coleman View Post
What DACs are we using? ESS9018? PCM170x ?
PCM1794A or PCM1792A -- the best DACs from TI with 132dB SNR (in one particular circuit as per its datasheet). Also it has one little function that helps solve the next problem:

Quote:
Originally Posted by Rod Coleman View Post
Volume control is another consideration - it's a shame to break up this single-current-path stage to accommodate a pot. Are the DAC-internal digital volume controls good enough yet?
Another unusual proposition is Reed relay switched resistor ladder. The DACs above have zero crossing signal, so if the volume has to be changed, relays wait for the zero crossing to switch, which eliminates a click due to switching. The best relay I could find switch in 0.25ms. The contacts are sealed in vacuum with an option of mercury coating to enable switching of very weak signals without loss or arcing. Combined with zero crossing it should be sufficient for silent volume regulation. Of course that requires a microcontroller to drive the relays. There is a much easier option for that in a single chip (such as TI's LM1972), but that means a FET switch in signal path at all times.
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Old 8th February 2013, 02:20 PM   #329
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Ok, but why are they difficult? It seems to me because of their low efficiency. Typical HiFi gear headphone outputs, smartohpones and similar stuff are not designed for such low sensitivity. So you get at lot of distortion when you try to listen to music louder, as could do with more sensitive headphones! Instead you can taylor your amp to the HE-500. I think 0.75W with 1.5% will be quite good. Actually I would be more worried about THD at low levels where you can pick it up easily, as I told before. From this point of view 3A5, 2A3 and similar are not so good....


75 ohm is the Zout of the amplifier not the secondary impedance. If you want use such secondary you still need to provide the 4P1L with the right primary impedance, hence you have to have the right turn ratio if you want 5K primary load. Using a 5K:36ohm transformer your Zout is in the region of 12-15 ohm.
I agree with what you are saying completly but there is always the chance that .75W isn't enough, what if in a year the new HE6 takes 2W. After building the "perfect" SET for Grados I look for this DHT build to provide exponetially more power so the latest and greatest will "work". I like playing with designs and research, but actually sort of desipse casework, so I am after something that will last


Quote:
Originally Posted by Rod Coleman View Post
Serving suggestion for 4P1L - 2A3 dc coupled.

to keep Anatoliy happy, the 2A3 anode current servos the operating current of the shunt cascode, to give dc-stabilised operating point.

2A3 needs -100V bias supply (10mA), but this does give a startup voltage of -100V, to be kind to the 2A3.

Darlington connected PNP shown. NXP Semiconductors PBHV9040Z assumed for main PNP. others are -T package (SOT23) - so yes, PCB needed for correct layout.

To keep the 400mV (pk) sensitivity, the gain of the shunt cascode is increased to 62, to allow for the low mu of the 2A3. It's the beauty of shunt cascode - you can set the gain independent of the stage's triode current.
The more I think about it we aren't going to come to consesus on a DAC, and its a bit of scope creep.

This 4p1l shunt cascode to 2a3/6c4c would fit all the critea of the original project definitions in the first page. It sims incredibly well and really I don't think the 2A3 is an expensive tube, the real secret is that the 6C4C with coleman filaments is a bit of an equalizer in that it is reported as good as the 2c4c. So we have a parts cost of well under $150 including all active devices not inculding chassis and iron.

Its powerful enough that one could use a huge range of off the shelf transformers with the 16 ohm tap and a parallel resistor to match impedance. I'll probably go for custom iron. Best bet is two 8 ohm secondaries. Run in parallel = speaker amp, in series center tap grounded gives a 32 ohm balanced headphone out.

All we need is a PCB

Last edited by regal; 8th February 2013 at 02:26 PM.
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Old 8th February 2013, 03:30 PM   #330
12B4A is offline 12B4A  United States
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Quote:
I agree with what you are saying completly but there is always the chance that .75W isn't enough, what if in a year the new HE6 takes 2W. After building the "perfect" SET for Grados I look for this DHT build to provide exponetially more power so the latest and greatest will "work". I like playing with designs and research, but actually sort of desipse casework, so I am after something that will last
It seems like your design goal should be for a case and layout that easily allows component changes and additions rather than for an omnipotent electrical circuit. Maybe that sounds a little smart-alecky but it is truly intended as something constructive and pragmatic about how you are approaching this mentally.
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