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.
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
GlassWare SRPP+ assembled board
and the board test setup - GlassWare SRPP+ kit on my test-bench
Measurements
Frequency response into 10k load
Frequency response into 32 ohm headphones
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
THD vs. amplitude at 1kHz, 10k load
19+20kHz CCIF IMD measurement at 1V
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
Maximum output voltage is very high, see 20Vrms sine plot
Square wave response, 10kHz,15Vp-p output, 10k load
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.
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.
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
GlassWare SRPP+ assembled board
and the board test setup - GlassWare SRPP+ kit on my test-bench
Measurements
Frequency response into 10k load
Frequency response into 32 ohm headphones
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
THD vs. amplitude at 1kHz, 10k load
19+20kHz CCIF IMD measurement at 1V
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
Maximum output voltage is very high, see 20Vrms sine plot
Square wave response, 10kHz,15Vp-p output, 10k load
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.