Phono preamplifier from Lineup & Fotios

[fotios]

Before 6 months, i asked from Lineup to help me in the design of a discrete operational amplifier implemented with BJTs. Lineup is known for his preference in FETs, so he proposed the use of them as base. Because my little experience with FETs, and that i haven't any such part in my stock (instead a lot of BC550C-560C) i asked him to turn in BJTs. Lineup was positive in my request, and so we immediately started the design with extensive use of simulator. The main drawback of discrete operational amplifiers compared to monolithic ICs is their big sensitivity to power supply ripple noise, so we focused our design on this above all. Our design is an upgraded version of D.Jensen 918, full of CCS and optimized to work with supplies from +/-12V up to +/-30V be it so un-regulated. The result was a discrete OPA with just 5mV residual output noise when is supplied from a +/-24V un-regulated supply and less than 1mV when is supplied from common 7824 - 7924 voltage regulators. In the picture is the prototype module for the tests, and in the schematic is the design. Its bandwidth limit is 500 KHz. I will talk later for further details of this discrete OPA and i will present some measurements. To the present, the most interesting part is the resistor R3 which establishes the LTP current and should be optimized according to the supply level and the arrangement of OPA. Resistors R4-R5-R7-R9 are optimized for supply from +/-20V up to +/-30V.

 

[fotios]

Part 2: The Lineup Phono Preamplifier

I have around 650 LPs. Some of them are in excellent condition and have excellent recording quality. Their dynamic range is very close to CDs. For example recordings of Joe Jackson, Magazine, Robert Cray, Kraftwerk... Recently i exchanged a power amplifier with a nice THORENS turntable equipped with a BENZ MICRO ACE-H M.C. head. Then, i decided to built a new Phono preamplifier optimized for this head.
I asked again Lineup for his help. Lineup, again, was positive in my request for collaboration. We decided to build the Phono pre around our discrete OPA refereed above. Moreover it was a big chance to we check its ability in a real project. After enough thought and study of different circuitries, we decided to build a two stage (per channel) unit with both stages in non-inverting arrangement. The first is a non-inverting buffer with gain X 2 and the second a non-inverting RIAA active filter with gain X 1000. Benz ACE-H is a high gain type M.C. head, with max output = 2.5mV. So, in the output we can get (2.5mV X 2) X 1000 = 5Vrms. Of course, the gain could be increased by adjusting the feedback resistors of first stage. The input filter is optimized for the concrete head. We did again, extensive use of simulator during design. In the pictures is presented the final unit. Everything is made at home. IMHO, Phono preamps, due to their extreme gain is the most difficult implementation in Audio. I made a box from 0.5mm thick copper foil to enclose the unit. It is a very good shield against environmental noises. In the output, by using my very sensitive headphones, i can hear only a very small hiss noise. My DSO it shows a noise of just 2 mVrms. In the beginning, we had problems with DC offset in output. After examination, we reduced the LTP current of the RIAA filter stage by changing resistor R3 to 2 KOhms and the problem resolved. Thanks to the trim-pots of each stage, the output offset can be reduced almost to zero. Thus the Phono preamp can be direct coupled to the next stage. There is no need for AC coupling capacitor.

 

[fotios]

Part 3: Lineup Phono preamp block diagram

Here is the block diagram of the unit. I have included in the schematic, only the external parts which form the input filter, the head compensation filter, the feedback network around the first stage and the RIAA filter components around the second stage. The discrete OPAs are shown in the first post. The only modification is noted inside the triangles and it is the R3 value, 1.1 KOhms for the first stage and 2 KOhms for the second stage. I omitted the supply unit (is mounted on the back of PCB) because is exactly as is presented in the data sheets of e.g. National semiconductors. We use common supply for both channels, composed from LM317 - LM337 adjusted at +/-22Volts. The only addition, is a combination of MJE172 - MJE182 connected in common base mode at the outputs of voltage regulators.

 

[fotios]

Part 4: Lineup Phono THD and Noise measurements

The measurements are obtained by using the Virtins MI Pro3.2 FFT software. The interface is an ASUS Xonar Essence STX sound card. I did everything is possible for the plots to not show "cooked". I don't use smoothing windows. And the vertical scale is in relative mode (dBr) because the absolute mode (dBFs) it shows better noise level.
The first plot is the best that i achieved. The second plot is that i achieved most times. The third plot is obtained with an inverse RIAA filter connected in the input of Lineup Phono pre. I use the "Hagerman" iRIAA filter.

 

[fotios]

Part 5: Lineup Phono pre Bandwidth

The same setup of Virtins MI Pro3.2 is used again, except the sampling rate which is changed from 48Ks/s - used for the THD+Noise measurements - to 192Ks/s which is the bandwidth limit of sound card (96KHz). In all plots, white noise is used as stimulus for the measurements. In the first plot is presented the frequency response of the Lineup Phono preamp. In the second plot is presented its frequency response with a Hagerman inverse RIAA filter connected in input.

 

[fotios]

Part 6: ASUS Xonar Essence STX performance

In the two plots, is indicated the performance of the sound card in "No Loopback" mode (that means loop with external cables from output back to the input) so as you can make your estimations on the above measurements. Virtins MI 3.2 setup remains the same.

 

[Mooly]

Excellent work as always Fotios and Lineup.

The output stage looks odd at first glance Q9 and Q10 for thermal compensation ?

 

[lineup]

Here is the block diagram of the unit. I have included in the schematic, only the external parts which form the input filter, the head compensation filter, the feedback network around the first stage and the RIAA filter components around the second stage. The discrete OPAs are shown in the first post. The only modification is noted inside the triangles and it is the R3 value, 1.1 KOhms for the first stage and 2 KOhms for the second stage. I omitted the supply unit (is mounted on the back of PCB) because is exactly as is presented in the data sheets of e.g. National semiconductors. We use common supply for both channels, composed from LM317 - LM337 adjusted at +/-22Volts. The only addition, is a combination of MJE172 - MJE182 connected in common base mode at the outputs of voltage regulators.

2 discrete opamps + RIAA (see blockdiagram)
The work put into this by fotios is Enormous
Thanks fotios

 

[fotios]

Excellent work as always Fotios and Lineup.

The output stage looks odd at first glance Q9 and Q10 for thermal compensation ?

Hi Mooly
Thanks a lot for the positive comments, you are very kind. Moreover we are players of the same... team . This discrete OPA is... the son of SIMULATOR . The sequence is: 1) You have a general idea 2) You put this idea in simulator 3) After optimization of circuit with simulator AC and FFT analysis, you draw the schematic and the PCB layout and 4) Voila monsieur the real circuit.
Simulator is a great tool. Because you wonder about Q9-Q10: In many discrete OPAs (included D.Jensen 918, Bryston DOA etc.) two diodes are tied between the emitter - base junction of output transistors for - as you correctly mentioned - thermal stability. I... asked simulator for this, and he replied that, if i replace the diodes with two transistors i could get a better THD performance of... 100nV in third and fourth harmonic . Because simulator is always right, i... followed its suggestion. Moreover, what is the extra cost for two more... itchy transistors?

 

[fotios]

2 discrete opamps + RIAA (see blockdiagram)
The work put into this by fotios is Enormous
Thanks fotios

Lineup is a modest man. The half of work is of Lineup.
Thank you friend for the collaboration on this project.

 

[Mooly]

Ah-So Itchy transistors !

You must have a good simulator to be on speaking terms... and it's when it answers back or disagrees you have to watch out

Very professional job as always...

 

[Salas]

Congratulations guys. Some questions and a suggestion also. What is the ein in nVrtHz, why its FR sags for a plateau below 500Hz? What is 0dB in VRMS on plots in post#4? You can try bypass the SCR input capacitors with 22n FT-1 at a point in future if you remember when shopping parts from ebay, cheap and makes a good difference in clarity there.

 

[fotios]

Congratulations guys. Some questions and a suggestion also. What is the ein in nVrtHz, why its FR sags for a plateau below 500Hz? What is 0dB in VRMS on plots in post#4? You can try bypass the SCR input capacitors with 22n FT-1 at a point in future if you remember when shopping parts from ebay, cheap and makes a good difference in clarity there.

Thanks Salas for your positive comments. As for your questions:
1) I haven't any method in my mind (to the present, i will try to find it) for measuring the ein (Equivalent Input Noise, right?) and my FFT software does not offers a such possibility from those that i know. I will ask Virtins. If you know a handy way to measure it, tell me.
2) If you mean the sag below 500Hz in "inverse RIAA" frequency response, that is just -1dBr. In other RIAA stages, i have seen a bump up below this frequency. It is probably owed in the 318μs (500.5Hz) pole of RIAA filter. You can see the same at the RIAA frequency response plot, where in the same point it starts a steep slope.
3) I have calibrated Virtins MI FFT according to my sound card output. All plots are UNWEIGHTED. For the FFT analysis, i use as stimulus a sinus of 1.6Vp = 1.13Vrms generated from the calibrated function generator of FFT software. So, i think 0dBr = 1.13Vrms
Sorry for my ignorance, but this is the first Phono preamp that i have build in my life! Lineup has more experience from me on this. By any way, i will try to remember your suggestion for the 22nF bypass cap. I think that i have some styroflex caps of such value. Are they suitable?

 

[Salas]

Hi. You can work back from your measured output RMS noise for an approximation. If its measured with a 20kHz window that is. See that video and reverse the calc. OK not a spectrum analyser, but its a method. Do a nice wide FFT 20Hz to 20kHz, input shorted. So you can see 1/F better also.
For that -1dB plateau below 400-500Hz something must be done because its a large area octave wise and its possible it will make it a bit weaker subjectively. There lies music's main power range. Can it be needing some easy to change value adjustment in some RC?
Yes the styroflex can be a possible test, don't know if it will work well, you do it and evaluate. I mentioned the Teflon because I have used the very 2u2 SCR with that FT-1 and it was a very good combination. For that I surely recommend. The THD is very good for 1.13VRMS.

 

[fotios]

Salas, i omitted something VERY important which is related with the dBr scale. You should know that my sound card can accept up to 1.6Vp input signal without risk of burn out of its ADC converter. So, i always adjust the output of MI function generator at 0.007Vp in the cost of the additional noise generated by sound card. This level, is multiplied by 2000 from the Phono preamp and is presented in its output as 0.007 X 2000 = 14Vp. That is very big, so i use a 10:1 voltage divider at the output of phono preamp to reduce it in 1.4Vp which is a safe value for the input of sound card. This procedure, is unfair for the real performance of Phono preamp. So, you can suppose as 0dBr = 1.4Vp = 1Vrms but this does not says nothing. Simply, the plots represent the worst case of preamplifier performance. For this reason i have included the two plots of sound card in "no loopback" mode and i use the dBr scale. It is very difficult case the measurements that i obtained with the sound card. With big risk to fry it again

 

[Salas]

You set 66dB gain for Benz Ace Blue? 40-46dB would be enough and you would throw out enough noise multiplication. Using no line preamp and low gain amp? (Ace its a very good cart by the way I have experience with the red one).

 

[fotios]

You set 66dB gain for Benz Ace Blue? 40-46dB would be enough and you would throw out enough noise multiplication. Using no line preamp and low gain amp? (Ace its a very good cart by the way I have experience with the red one).

The actual noise that i measured with my DSO (at least i have a such one!) it is just 1mV in output. With 2.5mV X 2000 = 5V the SNR is almost 73dB (very close to the value obtained from Virtins MI FFT software). How can i reduce the gain with 2 non-inverting stages in series? In the RIAA stage, it is impossible because the filter is precisely calculated with simulator. Salas, i spent 4 days for measurements. Phono stages are very difficult in tests. I can hear my blood pulse with my headphones, if i touch my hand on the cover plate!

 

[Salas]

You cut back the 5VRMS somewhere along the line with a volume pot I suppose, can't be having such an insensitive amp that would need all that input. So the noise floor remains in proportion. I was wondering about the ein so to know about low MC carts too since you got the gain. How do you like it subjectively with the Ace HiMC and your TT? Some good vinyl can be very enjoyable.

 

[fotios]

Power supply

Here is the schematic of the regulated power supply used in Lineup Phono preamplifier.

 

[fotios]

Actual Noise

With both inputs of Lineup Phono pre shorted, i tried to capture its real output noise with a Hameg HM502 DSO. It really was a difficult work, because the output fluctuates continuously between 0.4 and 1.2 mV and the lowest scale of DSO is 10mV. For this reason i got two photos, the first indicating the min value and the second the max value. At the same photos, you could see that the output DC offset is really 0 Volts.