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Old 7th December 2010, 03:15 AM   #1941
Joachim Gerhard is offline Joachim Gerhard  Germany
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I have now listened enough to say that this stage does not sound sharp ore spitty in any way. Quite the oposite. It is a smooth ride that reveils a lot of details, even at the low volume i am listening too. I get more and more the impression that many feedback stages that do not sound right have a problem with non ideal compensation. When i designed this stage i really made sure it works well in the time domain. I used a 10kHz squarewave for this purpose and i found interesting things. More to come.
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Old 7th December 2010, 07:33 AM   #1942
Salas is online now Salas  Greece
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The description of real music like smoothness, and complete mid bass presentation in low volume listening, rings familiar from my open loop. So you must have succeeded to come near to a non problematic loop. You made a single ended, high gain noise measuring instrumentation amplifier with EQ in essence. A difficult task.
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Old 7th December 2010, 08:31 AM   #1943
IngemarR is offline IngemarR  Germany
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It looks like SYN08's HPS5.1
Hps 5.1



Quote:
Originally Posted by Joachim Gerhard View Post
This is the schematic overview.
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Old 7th December 2010, 11:50 AM   #1944
Joachim Gerhard is offline Joachim Gerhard  Germany
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Hi Ingemar ! The Topology goes further back then the HPS 5.1. First there was the Self MC pre-pre back in the 90th. Then there was the Colins circuit in AudioXpress that uses a Fet.
Around the same time there was Burkhard Vogels BUVO that had a RIAA wraped around the input stage, again with bipolar input. Mr. Vogel split the gain though. His input stage has 47dB of gain and then he adds an adjustable linear gain stage. I also published a circuit in Linear Audio Volume 0 that has that kind of gain cell but set up as a symmetrical INA. Looking even further back Mr.Self must have gotten his idea somewhere and i found his basic topology in "Halbleiter Schaltungstechnik, Tietze-Schenk". I have only the latest issue 13. The basic circuit is on page 497. How old that idea is i can not say but Vogel also refers to Tietze-Schenk so i asume it has been there for many years. There is no new thing under the sun ( King Salomon ). What i claim is novel with my circuit is that i do the amplification and EQ in one stage. That saves me 2 gain stages compared to the HPS5.1 and one gain stage compared to the BUVO.
i also think that i have not lost quality in the process. There is around 77dB of feedback availlable at 20kHz so distortion should be superbly low. The Fets i use are optimised for Audio and have a very high Early Voltage for example plus low 1/F frequency plus low GR noise. I have other details in the circuit that give excellent PSU rejection and i drive the Fets at a certain operation points that avoids heating and gate current. I have published cascoded Fet stages here aplenty since the beginning of my thread 19 month ago. I must admit that i learned a lot about noise from Synn08 and Scott Wurzer, although it was not always a smooth ride.
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Old 7th December 2010, 12:08 PM   #1945
Salas is online now Salas  Greece
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A builder of 5.1 experienced VLF transients. Syn08 explained him why:Hps 5.1
It seemed endemic for such high VLF gain and an integrator servo by the explanation. Did you get any? Or you overcome that by utilizing less stages or due to another reason? Because you mentioned its destined commercial, no market would accept any transients for any time no matter how sort or start up. Dealers would perceive the phono as simply faulty. So its a crucial factor to check well it never occurs in yours, or add output delay relay, since it has been documented in an alike configuration phono. Just remembered that issue (non issue for DIY), so I thought I ring a bell. Sorry if you have covered that already.
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Old 7th December 2010, 12:26 PM   #1946
Joachim Gerhard is offline Joachim Gerhard  Germany
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Hi Salas ! I know problems like that and i call them motorboating. One stage i made had that problem. It was an earlier atempt of a Open Loop RIAA where i made a bias servo.
When i was cranking the system full i could see the woofer cones moving in and out at small amplitude with a low frequency.
No, the stage presented here does not have that problem but to stabilise the DC conditions
was not easy. A particular problem was the constant current source and i tried out at least 6 varieties from a simple Widlar current mirror to a double cascoded bandgap reference. The trick is to find a constant current source that has a high output impedance but is temperature stable in combination with the input stage. At an impedance of 50kOhm only 20nA produce an offset of 1V.
If this stage makes a nasty switch on transient i have to check. In commercial preamps i use a safety cap and a relay somewhere anyway but you are absolutely right. It is imposible to sell something on the High End market that is not functioning perfectly.
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Old 7th December 2010, 12:43 PM   #1947
Salas is online now Salas  Greece
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Those were the kind to look for with instruments during start up: Hps 5.1 The builder describes them as much alarming so you would have noticed if there was some already I believe. About the CCS on the particular application and type input cascode, yes it normally ''fights'' with it if not very special to it. That is why I prefer a relatively high resistor and high B+ clean PSU than CCS complexity and repeatability issues. Each PSU lends sonics of its own beyond noise susceptibility of stages no matter how good the PSRR in my limited experience, still the PSRR on noise has a highly measurable impact. I would go in lengths for a better PSU every time, even if it was only for feeding DC to top PSRR OP-AMPS, for its general impact beyond straight noise issues.
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Old 7th December 2010, 12:59 PM   #1948
Joachim Gerhard is offline Joachim Gerhard  Germany
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I think that builder could try reducing the 1kOhm resistor behind the servo but it is not my circuit so i hope the designer can help. The simple resistor solution is just fine but you also amplify the noise from the PSU that way. You also can not make the resistor value very high especially when you run the input stage on high bias. One way whould be to use a very high PSU voltage but i think this is not very elegant. In my circuit it whould not work. I really need the gain increase by presenting the input stage a high dynamic impedance. Anyway, Salas your PSU´s are excellent but i will not design more open loop phonos any time soon. Having gone thruw over hundred topologies including their varieties over the last 2 years i think i have covered the basics.
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Old 7th December 2010, 01:19 PM   #1949
Salas is online now Salas  Greece
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I wasn't proposing to adopt a load resistor since you already have arrived to a very well locking CCS for it. Just discussing other more traditional ways, that put the toll on the clean B+ of course, and increase care there. I mainly meant to say that I would make such an effort anyway for the PSU no matter the PSRR, because I feel it still makes its presence felt in different proportions in all circuits. But open loop and a few FETS VS hundreds of integrated transistors taking care of all problems in chips and active EQ loops are different beasts, maybe I am wrong, and would be much more impervious to PSUs. That 5.1 issue had been answered to the builder by the designer in the previous link. Those transient shapes I linked just to know the type they discussed, for reference when you will look for start up behaviour with instruments as you mentioned. I don't think they are there in yours, they would be very obvious, since you say you did not have any perceivable cracking noises. Did you have louder listening sessions by now?
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Old 7th December 2010, 01:27 PM   #1950
Joachim Gerhard is offline Joachim Gerhard  Germany
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Hi Salas ! I agree. Many successfull designers tell me that an amplifier is a modulated PSU and i got great results with your Simple Hypnotize, even in circuits with a good PSU rejection. As i mentioned here before i am not very experienced with PSU´s but i collected enough knowlage now to design my own. The CCS and the cascode bias i use are some kind of PSU anyway and i have already experimented with sofisticated cap multipliers. As a reference i will use battery supply and your newer version of the meaner shunts. We are in a process to do a PCB for that but it is not finished. If i need your help i will ask.
Louder session begins in 5 minutes. The system is warmed up already.
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