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cap coupling to interstage transformer or direct coupling?
cap coupling to interstage transformer or direct coupling?
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Old 23rd April 2018, 12:34 PM   #11
claudiomas is offline claudiomas  France
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the question its about interstage isolating coupling

try a bifiliar IT, no more of 250V between primary and secondary, this means avoid negative bias on secondary max 250V on primary and keep phase even a cheap 126C for 10mA max can give very good result

anyway you cannot affirm: "With IT coupling the frequency response and transient response get worse and hum/interference pick-up will increase."

but you can write: "in my experience i get bad frequency response etc"
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Old 23rd April 2018, 12:42 PM   #12
pieter t is offline pieter t  Netherlands
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Good quality interstage transformers, ratio 1:1, have wide bandwidth (depends also on source impedance, but over 100 kHz is no exception). This bandwidth is superior to the bandwidth of the output transformer, so it is not a restriction at all in a well designed amplifier.

Grids of output tubes like to see a low resistance path to mother earth, and that's a major benefit of interstage transformer coupling over capacitor - grid resistor coupling. One can discuss differences in sound, but that's done a million times already.
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Old 23rd April 2018, 12:47 PM   #13
deafbykhorns is offline deafbykhorns  United States
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Quote:
Originally Posted by claudiomas View Post
with IT coupling the frequency response and transient response improve and hum/interference pick-up decrease the sound improvement its great DHT triodes was made to work with transformers
I agree with claudiomas. Try a Hammond 126, they are cheap but are good transformers, bifilar wound as well.
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Old 23rd April 2018, 01:11 PM   #14
artosalo is offline artosalo  Finland
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Quote:
anyway you cannot affirm: "With IT coupling the frequency response and transient response get worse and hum/interference pick-up will increase."

but you can write: "in my experience i get bad frequency response etc"
It is not a question of my experience, it is a well know fact. Please show us one design where IT improves THD, frequency response and/or transient response.

In general, any transformer in the signal path degrade the performance compared to design without.
So why to add such components on purpose ?
Or is this intention to do nostalgic designs ?
(or is the answer the typical: sounds better...)

As a reference, Hammond 126A, look how it performs at 20 Hz driven with typical medium mu-triode or what sort of response it makes to 10 kHz square wave.

Hammond as a manufacture claims only this about 126-series IT's:
Quote:
These units replace coupling capacitor circuits for improved sound.
If it improved the performance in any way, why would Hammond not mention about this ?
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Old 23rd April 2018, 02:11 PM   #15
claudiomas is offline claudiomas  France
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i give up artosalo, go kipping use resistors and capacitors, or mosfet resistors and caps and be happy

thanks
PieterT.

anystereo, you can easily put grid choke on 45, LL1670 0.8mA. cheap great sound and no modifications

i love 45
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Old 23rd April 2018, 02:29 PM   #16
audiowize is offline audiowize  United States
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Quote:
Originally Posted by claudiomas View Post
with IT coupling the frequency response and transient response improve
Compared to what? The bandwidth extremes of an interstage transformer are nowhere near as good as a coupling cap or direct coupling. While I do agree that 100kHz could be reached on a parallel feed interstage transformer, gapped ones aren't going to do as well (maybe 30kHz up top, 30Hz down low unless you spend big money). This isn't necessarily the end of the world on a zero feedback amp with so-so output transformers, but if you do end up needing to use feedback or your OP iron has excellent bandwidth, the IT is going to be a bit of a bottleneck.

One way to get the low DC impedance path to ground for the grid of the output tube is to use a grid choke.

Quote:
Originally Posted by claudiomas View Post
and hum/interference pick-up decrease
No, IT iron is certainly sensitive to hum pickup. Cap coupling and direct coupling are not.

Last edited by audiowize; 23rd April 2018 at 02:34 PM.
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Old 23rd April 2018, 02:39 PM   #17
pieter t is offline pieter t  Netherlands
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Quote:
Originally Posted by audiowize View Post
The bandwidth extremes of an interstage transformer are nowhere near as good as a coupling cap or direct coupling.
Which is of no consequence at all in a good design! Are you listening with your ears or your oscilloscope?
I already mentioned the advantage of inductive coupling with low resistance path ("blocking distortion" anyone??)

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Originally Posted by audiowize View Post
No, IT iron is certainly sensitive to hum pickup. Cap coupling and direct coupling are not.
No issue in a good design and layout.
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Old 23rd April 2018, 02:43 PM   #18
pieter t is offline pieter t  Netherlands
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Quote:
Originally Posted by audiowize View Post
While I do agree that 100kHz could be reached on a parallel feed interstage transformer, gapped ones aren't going to do as well (maybe 30kHz up top, 30Hz down low unless you spend big money).
Utterly nonsense
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Old 23rd April 2018, 02:43 PM   #19
audiowize is offline audiowize  United States
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Quote:
Originally Posted by pieter t View Post
Which is of no consequence at all in a good design!
If the IT transformer is going to leave the amp down 3dB at 30Hz, and the OT is maybe down 1-2dB at 30Hz, I'd say the design isn't "good".

It's totally fine to love using interstage transformers, but saying that they have great bandwidth and aren't another variable for magnetic noise coupling is just perpetuating ignorance.
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Old 23rd April 2018, 02:45 PM   #20
pieter t is offline pieter t  Netherlands
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One more time Audiowize:
In an interstage transformer coupled amplifier with a good quality IT, bandwidth limitations are set by the OPT, period.
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