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Old 20th February 2011, 07:25 AM   #1
fotios is offline fotios  Greece
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Default Phono preamplifier from Lineup & 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.
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File Type: jpg NEW DISCRETE4.jpg (101.0 KB, 785 views)
File Type: jpg EAL discrete web.jpg (175.9 KB, 778 views)
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Old 20th February 2011, 09:30 AM   #2
fotios is offline fotios  Greece
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Default 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.
Attached Images
File Type: jpg LineupPhono_screened2.jpg (167.4 KB, 736 views)
File Type: jpg LineupPhonoFL.jpg (278.8 KB, 651 views)
File Type: jpg LineupPhonoFL1.jpg (331.9 KB, 640 views)
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Old 20th February 2011, 09:58 AM   #3
fotios is offline fotios  Greece
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Default 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.
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File Type: jpg Lineup Phono BLOCK.jpg (105.1 KB, 412 views)
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Old 20th February 2011, 10:41 AM   #4
fotios is offline fotios  Greece
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Default 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.
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File Type: jpg LINEUP PHONO BEST.jpg (271.1 KB, 347 views)
File Type: jpg Lineup FFT.jpg (161.5 KB, 109 views)
File Type: jpg Lineup iRIAA FFT.jpg (197.6 KB, 81 views)
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Last edited by fotios; 20th February 2011 at 10:45 AM.
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Old 20th February 2011, 10:58 AM   #5
fotios is offline fotios  Greece
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Default 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.
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File Type: jpg Lineup RIAA FFT.jpg (228.4 KB, 91 views)
File Type: jpg Lineup iRIAA.jpg (182.8 KB, 81 views)
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Old 20th February 2011, 11:04 AM   #6
fotios is offline fotios  Greece
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Default 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.
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File Type: jpg ASUS XONAR FFT.jpg (220.6 KB, 81 views)
File Type: jpg ASUS XONAR RESPONSE.jpg (276.0 KB, 62 views)
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Old 20th February 2011, 12:12 PM   #7
Mooly is offline Mooly  United Kingdom
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Excellent work as always Fotios and Lineup.

The output stage looks odd at first glance Q9 and Q10 for thermal compensation ?
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Old 20th February 2011, 05:12 PM   #8
lineup is offline lineup  Sweden
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Quote:
Originally Posted by fotios View Post
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
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Old 20th February 2011, 06:24 PM   #9
fotios is offline fotios  Greece
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Quote:
Originally Posted by Mooly View Post
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?
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Last edited by fotios; 20th February 2011 at 06:42 PM.
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Old 20th February 2011, 06:29 PM   #10
fotios is offline fotios  Greece
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Quote:
Originally Posted by lineup View Post
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.
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