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Fred Nachbaur SET amp: input capacitor
Fred Nachbaur SET amp: input capacitor
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Old 20th May 2013, 10:57 AM   #21
DF96 is offline DF96  England
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Just to clarify: carbon comp are OK to use in non-critical places if you have them and don't want to throw them away - but don't buy them specially. Elsewhere use carbon film or (better) metal film. In most places it doesn't matter which, but metal film may have lower noise and lower distortion. You probably won't be able to tell the difference, so don't get worked up about trivia. Getting the right circuit and doing grounding right (etc.) is far more important than component details. Some people worry about the latter because they don't understand the former - it becomes a displacement activity.
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Old 20th May 2013, 02:18 PM   #22
Soonerorlater is offline Soonerorlater  United Kingdom
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Fred Nachbaur SET amp: input capacitor
I'll keep my carbon comps for a guitar amp then. I was going to order some more from Farnell for a new build so I'll remove them from my shopping basket. Probably have the same values in carbon film already in my parts bin anyway.
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Old 20th May 2013, 07:15 PM   #23
rfengineer2013 is offline rfengineer2013
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DF96 makes an excellent point - some of the biggest problems in building amplifiers are ground-related hum.

Be sure to carefully plan your grounding scheme as you design the amplifier layout. The key here is controlling the ground currents to make sure they don't get into your amplifier by NOT depending on the chassis to be "ground".

One common method of avoiding ground-loop problems is to use a "star" grounding system that ties the grounds back to the power-supply and then to chassis.

There are good tutorials on this site (and on the internet, of course), but basically it boils down to some general principles:

1) Use isolated RCA jacks for the input connectors. I use jacks made by Rean (available from Mouser and other suppliers). These jacks come with little plastic shoulder washers that insulate the RCA jack from the metal chassis. If you use a non-conductive panel to mount your RCA jacks, you can use "regular" RCA jacks. Connect the RCA jack grounds into your system ground using good-sized wire (22-20ga is fine).

2) Connect your power ground back to the SECOND filter cap (C3 in the Nachbaur schematic). This ground is "quieter" than the C2 cap ground, which will be seeing nasty charging-spikes from the rectifiers.

3) Ground your system wiring to chassis at a single-point only, right where you bolt the green safety ground to the chassis.

I used a piece of 14ga solid copper wire as a bus for connecting grounds, soldering parts to the wire as it routes through the amplifier and connecting only ONE end of the bus-wire to the power supply ground point.

There are several excellent writeups on grounding to be found online, such as this one from Randall Aiken of Aiken Amps: Star Grounding

Ground-loops and hum are a big thing, easy to avoid with a bit of careful planning up-front and devilish to find later in the game.

One other major source of potential mischief is hum induced from the 6.3VAC heater supply. Be sure to tightly twist the heater wires (5-6 twists per inch), use 20-22ga wire, and keep them away from the input connetors. There is an excellent tutorial in the sticky-section of this forum on heater wiring with good photos. Since you have only one heater in this SPUD, you don't have to worry about how to interconnect the tube sockets.

What else can happen? Well, lots I suppose but these are probably the biggest problems in building audio amplifiers.

However, they are solve-able by doing a bit of research into the basics and trying to physically build things the way other people have successfully done. There are many fine examples of the work people have done on this forum and elsewhere on the 'net, you will quickly start seeing "good" layouts that will give you an idea of how to proceed with your design.

Good luck with the project!
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Old 20th May 2013, 07:37 PM   #24
rfengineer2013 is offline rfengineer2013
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One more suggestion: add a 150k (or so), 1W bleeder resistor across C3. This resistor will discharge the HV caps after you turn power off, but won't appreciably load the power supply.

It's a safety feature, not part of the signal path, and you can use some combination of your CC resistors for this (in case you've already placed your parts order).
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Old 21st May 2013, 01:58 AM   #25
TB is offline TB  United States
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Thank you all for your suggestions.
Sam, thanks for your input about grounding and idea of adding additional bleeding resistor. I will do that.
Grounding is still a little bit cryptic for me. I am going to do point-to-point circuit and I did already try to figure out grounding schema.
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Old 21st May 2013, 04:04 AM   #26
rfengineer2013 is offline rfengineer2013
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Quote:
Originally Posted by TB View Post
... Grounding is still a little bit cryptic for me. I am going to do point-to-point circuit and I did already try to figure out grounding schema.
Welcome to the club! Proper grounding can seem to be like black-magic, but there is some science behind it.

There's lots of variables involved, but the basic rule on grounding is to be aware of where signal and noise current is flowing and try to control it using wires. Also, avoid creating ground-loops, most commonly formed when grounds are attached to the chassis at multiple points.

Power Amplifier Grounding Scheme.pdf

This is a sketch of the grounding scheme I used on my Tubelab SE build. There are many ways to do this, the approach I took worked like a charm without any fussing. The amplifier noise floor is such that I cannot hear any hum or noise coming from my speakers - which is good enough for me.

The other aspect is that grounding is very layout dependent, there isn't any "cookbook" approach to grounding but if you study a few examples you will know more than many about how to best layout and ground your design.

One more note... Keep your power transformer oriented at right-angle to the audio output transformer(s). That is to say, orient the power transformer such that the laminations "point" at the audio output transformer windings. This will minimize coupling between the power tranny and the output transformers. Again, there's lots of examples on the forum and internet.

Let us know how the project turns out, too!

EDIT - The power tranny orientation might not be a problem with your parts, I just remembered you are using power transformers with horizontally-oriented frames. My comment above was more appropriate for when both audio and power transformers have vertically-oriented frames.
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Last edited by rfengineer2013; 21st May 2013 at 04:18 AM.
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Old 22nd May 2013, 12:54 AM   #27
TB is offline TB  United States
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Sam, thank you very much for your help. I will share the project building details. This is what I want to do. I am not electronically proficient and I do owe this forum a lot. The only way how I can contribute is to share my experience. It might take a while, ,but I will start soon to work on this project.
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Old 22nd May 2013, 06:14 PM   #28
sjs is offline sjs  United Kingdom
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An extremely good primer on grounding can be found in David Davenport's article on this site
Audio Component Grounding and Interconnection - diyAudio

whilst Rod Elliott's safety ground article is also worth a read
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
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Old 18th October 2017, 02:48 PM   #29
logotrikes is offline logotrikes  Australia
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Default Choke for the Miniblok power supply

JPS wrote...

***I also used a choke instead of R1***.

Pleased I found your entry.

I was going to ask about using a choke instead of resistor R1, does something good to the system. Don't know the terminology, smooths out ripples or somesuch. Dunno, just a beginner...

May I ask what size choke you recommend?

On another matter, Fred N used the 125CSE OT, presumably to rein in the cost. Hammond say this OT is 'general purpose' and not HiFi. Is that because it fizzles out below 100hz? If it did say go down to say 50hz or less, would it then be considered suitable for HiFi use?

It wouldn't blow the budget much more by going to one of Hammond's larger OT's to get 20 to 20k. Not in the spirit of Fred's creation sure but why not spend a couple of bob more...

Just thoughts. Collecting the various parts at the moment. Being in Oz I need 220-240v primary. Not easy to find here with 12.6vac 2A secondary, but I'm probably not asking the right questions or looking in the right places.

Jaycar have something similar, but 12v 2A secondary not 12.6v. Any good?

I'm sorry if either of my queries have been dealt with elsewhere..

I don't know what I don't know, hence the Noob questions...

Cheers, Martin
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Old 20th October 2017, 05:04 AM   #30
logotrikes is offline logotrikes  Australia
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Hi Folks,

I've started collecting the bits for Fred's 13em7 Miniblok. Ordered 4 Hammond power transformers and soon to be ordering the 125CSE OT's per schematic and Fred's description.

If this amp works ok would it be worth swapping out, down the track, the 125CSE for the much better and much more expensive Hammond 1638SEA. I understand that there is no substitute for good iron on the OT's but not sure if a "budget" amp would be worth another AUD500 per pair of the 1638SEA (which incidentally is also suitable for 211 & 845-different league altogether)

I'm like Sgt Schultz, I know nothing, or very little anyway..

Cheers for any input guys, Martin

Last edited by logotrikes; 20th October 2017 at 05:13 AM. Reason: wrong grammar
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