Grant Amplifiers

Home made HIFI

This is the documentation of a preamplifier performance data versus a high performance 6sn7 SRPP with current sink cathode follower output. N.O.S. matched section tubes used throughout the experiment.

Conditions: 10 MA of current, 300 V one choke supply.

All test data at 1 kHz. Identical results at other frequencies.

Output power of 2Vrms raw 2.92 hertz resolution, distortion tests:

Data of circuit mentioned above:

2nd: 0.056% - produces a warm sound and loss of details and clarity

3rd: 0.005

My circuit:

2nd: 0.022

3rd: 0.031

4th: 0.005

My circuit with optimization of input bias: 

2nd: 0.013 - super clean sound

3rd: 0.016 - super clean

4th: 0.008

Noise from the power supply and mains: -58.1dBr at 120 hertz; -67.5dBr at 60 hertz, A-weighted to -74.1dBr at 120 hertz, and -86.1dBr at 60 hertz. Actual measurements show 3 MVrms at the output, which combines the entire noise floor. This is perfectly acceptable.

Higher frequency noise patterns created at the volume control are well controlled at -75 dB and better for frequencies at 2.2 KHz, 4.5 KHz 6.75 KHz 9 kHz over this: 11 KHz is -83 dB and 13.5 KHz is -86.2 dB. This is with volume to 2/3 turned up. This noise goes very low if the volume is turned to haft position.

The distortion levels at 1Vrms are very low:

2nd 0.02 %

3rd 0.004

4th 0.007

5th 0.003

6th 0.003

The greatest thing about design is that it will not create any harmonics close to the signals, which are likely caused by feedback inter-modulation effects which smear the texture of sounds. You can drive any combination of signals in the preamplifier and they will be kept completely intact. The only inter-modulation effects are within distortion itself and contained to a single mirror effect at the second harmonics (see point A below).

IMD tests:

Near identical frequencies at 0.7 Vrms:

2nd 0.018 x 2

3rd 0.006 x 2

4th 0.009 x 2

(POINT A): We need to add one inter-modulation summing effect of the 2nd harmonic distortion of around 0.018%. The inter-modulation harmonic is the lowest 2nd harmonic mirrored over the highest 2nd harmonic.

Typical -3bB points for 150KOhm input impedance (recommended): 35 Hz, 40 kHz. Please advise me if you use a lower input impedance amplifier to change the output capacitor.

Conclusion: This is the only preamplifier with such a low level of distortion. Better results can only be obtained with a transformer coupled common cathode preamplifier with feedback. Because the preamplifier can be inserted inside the feedback loop of a stereo amplifier of 2 stages ( driver and output) which already suffers from mirror effects IMD there is no reason to separate the components from one another. Even the best preamplifier add some distortion or in worst case close IMD defects. Most of the distortion is eliminated by placing the preamplifier stage inside the main feedback loop of an amplifier.

Copyright 2014 Amplificateurs Grant