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GlassWare SRPP+ tube preamp kit review and measurements

pma

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#1
Review and Measurements of GlassWare SRPP+ kit

After reading the Bottlehead Crack Headphone Amplifier Kit Review here at ASR
https://www.audiosciencereview.com/...d-crack-headphone-amplifier-kit-review.15714/ I have decided to review and show my older measurements made on the GlassWare SRPP+ kit that I built in 2011. I was quite surprised by the high level of spurious mains components multiples seen in Amir's Bottlehead Amplifier kit measurements, because I did not remember it in the GlassWare kit. I agree this may be a difficult point in the tube design, especially with regards to PCB design and wiring of the amplifier.

The original GlassWare kit is is not in production anymore, however there is a very similar successor kit https://glass-ware.stores.yahoo.net/srliam.html

The link shows circuit diagram of the kit, it is possible to choose part values according to tubes used and supposed purpose of the circuit (gain, load, supply voltage).

Description

The kit PCB contains a high-voltage power supply, a heater regulated DC power supply that can be set to 6V or 12V and 2 sockets for double-triodes, I used ECC82 which are equivalents of 12AU7.

srpp1_pcb.jpg

View of the SRPP+ kit PCB
The PCB is extra thick, 0.094 inches double-sided FR-4 PCB of a very high quality. Dimensions are 6” x 6”. Next image shows an assembled board

foto_srppplus.jpg

GlassWare SRPP+ assembled board

foto_test_srppplus.jpg

and the board test setup - GlassWare SRPP+ kit on my test-bench

Measurements

Frequency response into 10k load
srpp freq.png


Frequency response into 32 ohm headphones
freq_tubehead_32ohm.png

Output impedance is high, so the frequency response is modulated by the impedance curve of the headphones. However, distortion does not change much with load, so one may have a “sound-processor” with elevated bass.

THD and THD+N at 1kHz, 1V, 10k load
srpp_thd1_1V.png

THD vs. amplitude at 1kHz, 10k load

thd_ampl_srpp.png


19+20kHz CCIF IMD measurement at 1V
srpp_ccif_1V.png

2nd harmonic (difference tone 1kHz) and 3rd harmonic ("skirts") can be seen.

Output noise

The noise measured was -83dBV(A). Taking into account as high output voltage as 20Vrms, the S/N related to maximum output would be 109dB(A).


Maximum output voltage
srpp_20Vrms.png

Maximum output voltage is very high, see 20Vrms sine plot


Square wave response, 10kHz,15Vp-p output, 10k load
10kHz_sq.png

The response to 10kHz square is very nice, without overshoot and with a very short rise time.


Conclusion

GlassWare SRPP+ kit is nicely built, with a good circuit and PCB design that reduces hum and mains spuriae of the tube preamplifier. Distortion, as expected, is nothing to call home about. However, frequency response into resistive load is flat and wide, with -3dB corner frequency near 300kHz. This corresponds with a nice square response. Output voltage is very high, up to some 20Vrms. However, the circuit is best to be used for output voltages <= 3Vrms.
 

raindance

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#5
John Broskie, the designer, has some really good ideas about improvements to tube designs and has invented a new topology, called Aikido, that has excellent power supply noise rejection.
 

DualTriode

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#6
My biggest gripe about tube circuits and TubeCad stuff in particular is power supply noise. You obviously know what I am speaking of as you only show the FFT above 500Hz.

Come on show us that FFT with 60 Hz, 120Hz, .... 420Hz stuff.

Thanks DT
 

solderdude

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#7
My biggest gripe about tube circuits and TubeCad stuff in particular is power supply noise. You obviously know what I am speaking of as you only show the FFT above 500Hz.

Come on show us that FFT with 60 Hz, 120Hz, .... 420Hz stuff.

Thanks DT
He lives in Europe so would be 50, 100, .... 400 Hz stuff.;)
 
OP
pma

pma

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Thread Starter #8
My biggest gripe about tube circuits and TubeCad stuff in particular is power supply noise. You obviously know what I am speaking of as you only show the FFT above 500Hz.

Come on show us that FFT with 60 Hz, 120Hz, .... 420Hz stuff.

Thanks DT
This is no problem to show. The attached plot is for 2.82Vrms output. Compare to
https://www.audiosciencereview.com/...d-crack-headphone-amplifier-kit-review.15714/
and you will see that the GlassWare kit is much better (or my test wiring is much better). I believe you can extrapolate and compare the results. If it was as bad as the Bottlehead kit, you would see the mains multiples spread through the whole frequency range.

/This is the Bottlehead as a comparison https://www.audiosciencereview.com/...dphone-amplifier-audio-measurement-png.80840/
tubepre_dist_9dBV.png
 

DualTriode

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#9
Broskie uses B+ and H+ regulators on that PCB, that helps a lot. The rectifiers switching on and off still put a moderate amount of junk into the circuit because of proximity. Put the regulators into a remote enclosure that helps attenuate a few dB more.

Nice build, nice photos, nice test plots.

Tubes are fun
DT
 
OP
pma

pma

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Thread Starter #10
It is a rainy day, I have some time so I have decided to add some more measurements of this tube preamp. First, I re-measured the old plots and got the same results, so there is not much reason to post them. Then I made some measurements into 50 ohm load (resistive) and the results are not completely intuitive. First, I need to say that the circuit's output impedance, with components used, is unusually high if we work only with solid state circuits. The output resistance measured is 4821 ohm. So, after loading with 50 ohm, the output voltage drops 96 times. In my case, from 6.82Vrms to 0.07Vrms and the tube circuit acts rather like a constant current source. And, starts to act like SRPP (and not like a single stage with active load). Let's see what happens with the plots. Following are distortion at 1kHz, distortion vs. frequency and amplitude frequency response, all measured into 50 ohm load. Unloaded output voltage would be 6.82V, this falling to 0.07V when loaded with 50 ohms.

srpp_thd1k_50ohm2.png

THD+N at 1kHz into 50ohm load

srpp_thdfreq_50ohm.png

THD(+N) vs. frequency into 50ohm load. See also plots for 2nd and 3rd harmonic components.

srpp freqresp 50ohm.png

Amplitude frequency response into 50 ohm load


So, the circuit works quite well into low ohmic load like 50ohm. Of course, in case of complex load (inductive and capacitive impedance components) the frequency response would be modulated by the impedance used.
 
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