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GG preamp, why is it uncommon?
GG preamp, why is it uncommon?
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Old 11th January 2018, 09:53 PM   #1
geezertron is offline geezertron  United States
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Default GG preamp, why is it uncommon?

I'm spending my evenings trying out every idea for using a 4HA7 twin triode in a pre-amp, basically an "AU" and "AX" in the same bottle.

I ran across the attached circuit at:

Real Grounded-Grid Aplifier

With the values of B+ and resistors it came in at 20dB gain, vs 19dB as stated in the article, but I'd consider this pretty much spot on equal given the different tube and voltages.

It's a wonder. The low-ish (for a tube amp) input impedance matches the intended application as an "AUX IN" level amp, and reduces hum pickup. No signs of oscillation, and it actually produces usable gain (from an RF standpoint I guess ;-) at 600Khz. Insane bandwidth basically. Since the forward coupling and feedback are both DC there's no phase shift at all, the input and output lie square on top each other. Basically lot's to like.

So why isn't it more common, or am I missing something when I search for similar circuits?
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Old 11th January 2018, 10:34 PM   #2
Lingwendil is offline Lingwendil  United States
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GG preamp, why is it uncommon?
Low input impedance and lack of direct connected input (at least, without special circuitry or consideration) are my opinion the main things keeping people away.

I've used them at the output of current output DAC circuitry (PCM1794) using submini tubes in the past and it was a very simple yet fantastic solution, even if a bit unorthodox.


Fyi that diode does nothing once the tubes are warm, but is worth leaving in circuit, or replacing with a neon lamp.
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Last edited by Lingwendil; 11th January 2018 at 10:37 PM.
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Old 11th January 2018, 11:13 PM   #3
Johan Potgieter is offline Johan Potgieter  South Africa
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As Lingwendil said, not a very conveneint topology in audio: Uncomfortably low input impedance, ditto very high output impedance. Often one requires a second tube to make things practical, with which one can in the end do more useful things with different twin-tube circuits.
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Old 11th January 2018, 11:36 PM   #4
Wavebourn is offline Wavebourn  United States
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GG preamp, why is it uncommon?
A nice amp, actually, with a good feedback, hence low distortions and output resistance. You can also cascode it with a JFET underneath, for more gain and higher input impedance. :-)
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Old 11th January 2018, 11:41 PM   #5
Eli Duttman is offline Eli Duttman  United States
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Up to now, the discussion has been about a true grounded grid stage. Then there is what Bruce Rozenblit called grounded grid, which is "really" a buffered diff. amp. I'm providing my take on that sort of circuitry. With zero loop NFB, the setup is HIFI, and then some, linear.
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Old 12th January 2018, 05:02 AM   #6
geezertron is offline geezertron  United States
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Some discoveries:

Mostly looked at the input impedance tonight. Measured (very) roughly as the value of the series resistance that halves the output with a constant input V, the amp as-is has 7K input impedance.

If you split the anode load on the gg side and bootstrap it from the output of the cathode follower side with a large cap (I used 11.5uF) it doubles the input impedance to 14K, and increases the gain about 10%.

On the other side of things, if you return the cathode resistors on both sides to -17V the input is pulled down to 0V and can be directly coupled. I amused myself for a couple of minutes looking at perfectly square 1 Hz square waves, but can't see the practical utility.

So far so good, I'm liking this little amp more and more...
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Old 12th January 2018, 09:42 AM   #7
DF96 is offline DF96  England
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Yes, bootstrapping the GG anode will help. Not only does it raise input impedance (to nearly reach the value of the cathode resistor) but more importantly it also linearises the input impedance. Not an issue if you are driving it from a headphone socket, but could matter if the source has higher impedance.

The downside of bootstrapping is that it raises output impedance too.
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Old 12th January 2018, 08:13 PM   #8
kodabmx is offline kodabmx  Canada
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Good info here: Real Grounded-Grid Aplifier
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Old 13th January 2018, 07:22 PM   #9
6A3sUMMER is online now 6A3sUMMER  United States
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When the 12AX7 is warming up, it puts current back into the signal source (10V charges the 3 uF cap).

When the 12AU7 is warming up, it puts current into the output. 103V charges the 1 uF cap. Without any other DC path to ground, the time constant is 1uF x 1 meg Ohm = 1 second. That means that the output will be at about +5V after 5 time constants (5 seconds).
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Old 13th January 2018, 07:35 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by 6A3sUMMER
When the 12AX7 is warming up, it puts current back into the signal source (10V charges the 3 uF cap).
Yes. The input ground leak resistor may need to be a bit smaller.

Quote:
When the 12AU7 is warming up, it puts current into the output. 103V charges the 1 uF cap. Without any other DC path to ground, the time constant is 1uF x 1 meg Ohm = 1 second. That means that the output will be at about +5V after 5 time constants (5 seconds).
No. The diode ensures that the voltage is set before the 12AU7 warms up. The instantaneous output voltage on switch on could be 100V or more. It will then decay. By 5 time constants it will be around 700mV or maybe slightly more. A smaller ground leak, combined with a slower HT/B+ rise will avoid this problem.
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