Inspire 2030 UX-201 DHT preamp

Someone told me about this the other day. So, I emailed Dennis and asked him to build one with a 5Z3 instead of a 5U4G. I have his LP27a preamp and 45/2A3 amp. The 45 is begging for a DHT to play with.

 

MrEd

Senior Nobody
That is very good price for what he is making.
I have been wanting to build an A01 pre for a while.
 

paulbottlehead

Active Member
The big benefit of a DHT in a linestage is the overall linearity of the DHT itself. This means very low distortion without resorting to feedback provided adequate loading impedance can be used.

The disadvantages are numerous. DHTs are noisier (regardless of how quiet the filament power is), require individual quiet filament supplies, are susceptible to microphonics, and typically are in that middle ground where they really need help driving a linestage output. For an 01A, you're a bit stuck as they aren't well suited for driving transformers, so you'd want to use another triode after them or a mosfet. (There's a rather lengthy thread on DIYaudio about this)

In the Inspire preamp, there are just 01As and power supply tubes, so I would wonder what the output impedance is. I would suspect it's up around 8K but there aren't any published specifications or any real information in the listing about the preamp, so YMMV.
 
The big benefit of a DHT in a linestage is the overall linearity of the DHT itself. This means very low distortion without resorting to feedback provided adequate loading impedance can be used.

The disadvantages are numerous. DHTs are noisier (regardless of how quiet the filament power is), require individual quiet filament supplies, are susceptible to microphonics, and typically are in that middle ground where they really need help driving a linestage output. For an 01A, you're a bit stuck as they aren't well suited for driving transformers, so you'd want to use another triode after them or a mosfet. (There's a rather lengthy thread on DIYaudio about this)

In the Inspire preamp, there are just 01As and power supply tubes, so I would wonder what the output impedance is. I would suspect it's up around 8K but there aren't any published specifications or any real information in the listing about the preamp, so YMMV.

AFAIK, Dennis does not publish much in the way of specs. Dunno why. FWIW, the pre can also run 71a tubes, and those that have one report that it's dead quiet.
 

paulbottlehead

Active Member
Play a 60Hz tone into the input and connect your meter to the output set to AC volts. Write down the output voltage you see, then touch a 1K resistor across the meter probes and write down the new AC voltage, which will be lower. Throw said numbers into this calculator:

Measuring input impedance and calculating output impedance calculation calculate audio amplifier op-amp how to measure impedance voltage gain total loudspeaker speaker microphone resistance resistor bridging matching test load loudspeaker source resistance - Why input impedance is in parallel and output impedance is in series? - sengpielaudio Sengpiel Berlin
 

paulbottlehead

Active Member
If you have a phone, laptop, tablet, or other device with an internet connection and a headphone jack, you can play a 60Hz tone off a website through an 1/8" TRC to RCA cable. If you have a CD player, you can get a test disc with tones on it. If you have a media server or computer hooked up to a DAC, you have no excuses!

The resistor doesn't have to be 1K, but I would recommend something between 500 ohms and 5K.
 
The pre arrived Saturday, it is dead quiet and as RPMac said, human voices are spooky real. I'll post again after it settles in.
Like Paul says, DHTs have what I would rather call “issues“. I’ve had tubes that when first installed are rather microphonic, but the more I played them, the less it was a problem. I have my amps on a heavy, solid marble top piece of furniture…I think this may help with some of the noise issues.

Enjoy!
 

MrEd

Senior Nobody
Thank you APaul for the instruction, I have been wanting to know what my preamp is.
I do like my 2P29L pre very much.
I hope try the 01 some day.
 
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