John Curl's Blowtorch preamplifier part II - Page 558 - diyAudio
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Old 9th August 2010, 04:05 AM   #5571
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I suspect that a complete 're-think' of the design would improve things, rather than adding more stages.
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Old 9th August 2010, 04:49 AM   #5572
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Quote:
Originally Posted by hitsware View Post
>Only thing that really works to stop the hum is
>the 0.1mH ferrite bead choke.
>So, I think you might be right Hitsware,
>about the demodulated high frequency
>noise causing the hum problem.

That is one place where jfets shine ....
Don't demodulate like bipolar ....
Obviously (IMO) because they are reverse biased ...
Will jfets work in your circuit ! .... ( maybe just for the input ) ?
I could do jfets, if I can find p and n channel ones. I need positive and negative voltage rails.

I used the 2n3906 since I had them on hand and they're cheap and it has a nice complement in the 2n4904. My goal is to build a cheap and simple design with the best possible performance. I want it to be shunt.

It's going to power a jfet single ended preamp design, maybe cascode. Haven't worked out the details on that yet.
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Old 9th August 2010, 05:03 AM   #5573
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Quote:
Originally Posted by Joachim Gerhard View Post
The other John talkes about demodulation so i thought a cascode could shift that problem up in frequency. I also found that cascodes seem to have better DC stabillty then transistors that are left alone. That depends then mostly on the bias scheme of the cascode transistor. I mostly use Leds and RC filtering or low noise precission references when i try to do something better.
I'm also going to experiment more with the active, feedback shunt. It was cascode, but not the current source. It was the same.

John

Quote:
Originally Posted by John Curl
I suspect that a complete 're-think' of the design would improve things, rather than adding more stages.
Thanks John!

I don't why people here dislike my RF choke fix to the problem. It seems to work good, time will tell. But, I agree, it could be better and I'm not done yet.

I should fix my noise amp first. It's Dennis Collin inspired but I had some problems when constructing his exact circuit, entirely my fault I'm sure. Probably wired something wrong. Mine seems to be a noiser that his. His output noise was -135dBv I think. Mine just -120dBv, -125dBv on a good day.
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Old 9th August 2010, 06:22 AM   #5574
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The source of your problem seems to be the active current source. If you have fixed it, then no problem, if not, complicating an already primitive circuit, isn't going anywhere. With such circuits I usually find that a film cap bypassing the input is really important. I have found that 5000 pf mylar is OK, for example, and probably removes a lot of TVI that is picked up.
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Old 9th August 2010, 08:08 AM   #5575
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DIY consulting thread?
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Old 9th August 2010, 11:40 AM   #5576
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Quote:
Originally Posted by john curl View Post
The source of your problem seems to be the active current source. If you have fixed it, then no problem, if not, complicating an already primitive circuit, isn't going anywhere. With such circuits I usually find that a film cap bypassing the input is really important. I have found that 5000 pf mylar is OK, for example, and probably removes a lot of TVI that is picked up.
I agree, complicating a circuit that doesn't work isn't helpful. My circuit seems to work good though. But, I think, I'll take Hitsware's advice and turn the current source around and switch to NPN devices so that only the collector "sees" the RFI,TVI noise. I'll try the 5000pf mylar cap too. I don't have mylar on hand ... is that important or just adadequate for the job?

I'll start another thread if I decide to keep discussing this, so I won't disrupt your thread.

John
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Old 9th August 2010, 11:42 AM   #5577
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5000pF Mylar is a near equivalent to 4n7F MKT, MKS, Polyester.
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Old 9th August 2010, 12:14 PM   #5578
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Quote:
Originally Posted by john curl View Post
Johnferrier, I am fairly sure that either makes a pretty good recording, by today's standards. However, WHY would someone buy a record for about the same amount as these recorders cost today? Collector's madness?
No matter what the ads say, or public opinion, there is a difference between what we did to record audio, and what many people have available to them, today.
Still, I bought a Sony radio, yesterday, on Dick Sequerra's recommendation. I couldn't resist the price/performance ratio, and it is always good to try new technologies, but keep in mind that IC's still don't do what tubes or discrete circuitry does. I wish that it did.
Hi John,
Did you get the Sony XDR-F1HD? : http://theaudiocritic.com/plog/index...e&articleId=35

That tuner shows the (future) promise of high-speed RF DSP, and Sony utilized that to great advantage, making the IF bandwidth dynamically variable depending on signal strength and adjacent-channel interference. From an engineering standpoint, it is the first tuner to offer those benefits. A a broadcast engineer, I can appreciate how much benefit can be obtained from a correctly aligned IF filter section, and the DSP alignment seems to offer many improvements over a multiple ceramic-filter IF selection topology, if only from a correct alignment perspective.

A fellow amateur operator, Brian Beezley spent quite a bit of time dissecting that tuner and suggests modifications:
Sony XDR-F1HD

From a purely technical perspective, the tuner is an amazing feat, but after auditioning one I did not buy it (I may still if Bay offers one up to me cheap). As a radio engineer I know what the sound of processing used in the major stations is like. The station I set up and maintain is using an Orban 8500 set up to do JUST AGC. No individual frequency band processing at all. So, minus the long-term dynamic range modulation, it sounds pretty damn transparent. When listening to it at a good signal strength (70dBu reception area) I thought that for my tastes (emphasis on MY) it sounded grainy. Some sort of grit or grunge is added to the signal, compared to the best analog tuners I own. On the other hand, it's RF performance is nothing short of amazing, and comes close to, or equals the best analog tuners.

I think that this Sony tuner represents the first wave of tuners utilizing digital IF processing to replace expensive and difficult to align analog IFs based on ceramic filters, and like the CD format, will get better with time as the technology allows for higher-speed DSPs. I don't know what IF oversampling is possible with the NXP TEF6730/SAF7730 chipset, but I'm sure future generations will be capable of better performance.

Until then I will continue to use my Kenwood 600T at home. It may be old (1980), but it sounds extremely sweet with a decent signal.

Howard Hoyt
CE - WXYC FM-89.3
UNC Chapel Hill, NC
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Old 9th August 2010, 01:42 PM   #5579
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Johnloudb - there are a number of excellent regulator circuits already designed and working. Some are discussed on this forum. Some have pre-made pcbs available, some are ICs (three pin variety?) that you can just use... I'd start with that myself.

John Curl and everyone else - if you didn't spot it already, a bit of
audio history:

Big Electro Research Class A amp

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Old 9th August 2010, 04:23 PM   #5580
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Quote:
Originally Posted by hhoyt
Hi John,
Did you get the Sony XDR-F1HD?
I know you meant the other John, but I did buy it. And, it worked good for FM, however, whenever it finds and HD radio signal it switches automatically to HD radio. I read the manual and can't find a way to disable HD radio. If someone can tell me how, send me a PM please ... I cannot even listen to it in HD radio mode.

Quote:
Originally Posted by bear
Johnloudb - there are a number of excellent regulator circuits already designed and working. Some are discussed on this forum. Some have pre-made pcbs available, some are ICs (three pin variety?) that you can just use... I'd start with that myself.
Yeah, I've seen some nice ones here. I just enjoy designing my own stuff ... that's the fun part for me. I've designed a few regulators and preamps and my ears have no complaints.

John
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