Hi everyone !
I am planning to build a preamplifier with DHT. The fact is that I need at least 10dB of gain.
I am interested by the 10Y, 801A, 101D.
But with those tubes I can't use step-down output transformer because I need the entire gain of those tubes.
I can use step-down transformer only if I use two 10Y or 801A etc..
I feel it's not a good solution to use two tubes when only one tube can do the job.
I can also look after another candidates, the 20B, with mu of 20, I can have the necessary gain and step-down transformer to have really low output impedance.
I am wondering if it's really an advantage to use high mu tube to have low output impedance ?
Why some people are using step-down output and use step-up at input of the amplifier ? Why it seems so important to have low output impedance ?
Why some people prefer use two gain stage to have low output impedance ?
My amplifier have 47k input. But anyway, I can change it.
Thank you
I am planning to build a preamplifier with DHT. The fact is that I need at least 10dB of gain.
I am interested by the 10Y, 801A, 101D.
But with those tubes I can't use step-down output transformer because I need the entire gain of those tubes.
I can use step-down transformer only if I use two 10Y or 801A etc..
I feel it's not a good solution to use two tubes when only one tube can do the job.
I can also look after another candidates, the 20B, with mu of 20, I can have the necessary gain and step-down transformer to have really low output impedance.
I am wondering if it's really an advantage to use high mu tube to have low output impedance ?
Why some people are using step-down output and use step-up at input of the amplifier ? Why it seems so important to have low output impedance ?
Why some people prefer use two gain stage to have low output impedance ?
My amplifier have 47k input. But anyway, I can change it.
Thank you
What cable capacitance is there between the preamplifier and the main amplifier? The capacitance will form a low-pass with your preamplifier's output impedance. If the bias current is low and the cable capacitance high, you might also get slewing when the signal has a high rate of change (which most audio signals have not).
The hybrid mu-follower (aka gyrator) can provide very low output impedance (1/gm of top FET) and the gain of the low/mid-mu DHT. Also you can implement filament bias in the DHT to get the best sound out of it. Some high filament current DHTs will not work. the VT25/10Y can be implemented with filament bias for a preamp stage. I have it on my system and is one of the best preamps I’ve ever built over the years. Have more than 6 DHT preamps active these days!
801a has about 5k plate resistance -at typical 18..20mA anode current-.
10Y/801a 1:1 OPT loaded preamp has -about- 5.5k output impedance (at 1kHz) such similar -high- anode current.
There is no change even 20kHz or higher frequency.
Preamp tubes -with such high plate resistance- at tiny (few mA) anode current mostly unable to drive device (cable or amplifier) with high capacitance (at high frequency), so step-down transformer (or another method -for example CCS or gyrator-) is necessary to use.
10Y/801a 1:1 OPT loaded preamp has -about- 5.5k output impedance (at 1kHz) such similar -high- anode current.
There is no change even 20kHz or higher frequency.
Preamp tubes -with such high plate resistance- at tiny (few mA) anode current mostly unable to drive device (cable or amplifier) with high capacitance (at high frequency), so step-down transformer (or another method -for example CCS or gyrator-) is necessary to use.
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