DIY Audio Projects Forum

One other time I found hiss was when the filament voltage was too low. I'll assume that has been checked.

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Projects:”Image” - Low Profile 100W Class D amp | ”fiih” - 150W Holton inpsired power amp | "Precision" - 300W Class AB Holton power amp | ”Gothic” - FET Push-Pull headphone amp

Thanks Matt. i'll try moving the output capacitors to other spaces in the chassis using shielded hookup wire to see if I can make some room next to the tubes. That way I can increase the distances between components around the tube sockets, and space things more in general. I'll post with the results.

mwhouston: The heater voltage is 6.6 Volts. I will add some resistance to each leg to get it down to 6.3 Volts and let you know what happens. This may be a contributing factor to the hiss. Thanks!

Matt: I rearranged the circuit and moved the output capacitors away from the area above the tube sockets. They are now located next the the power supply capacitors. This gave me more room around the tube sockets, and I was able to arrange the wiring so that the wires and component spacing are now closer to what is normal for my builds. It's still on the tight side, but there are gaps between all components and wiring with nothing touching. It's hard to tell from the photo, but things are spaced from top to bottom as well. I tested the preamp in my system, and found the hiss noticeably reduced, but still objectionable, with exaggerated sibilants. Also, the hiss increases as volume is increased and then goes down at full volume. The reduction in hiss caused by the modification tells me that the close proximity of wiring and components was, and still is, one of the causes of the hiss as you suggested in your last post. Do you feel that given the required tight layout, it would be prudent to add a grid resistor to the 12AU7 cathode follower tube in order to help get rid of oscillation? If so, what value would be advisable for that resistor? I have attached a photo of the modified interior for your consideration.

Hi everyone. I have a few questions that stops me from finally starting this build. First, the power supply voltage. Is there a simple way to bring the original 4S PSU B+ from 250V to 300V ? I already have all the parts to build it to the specs of the original 4S. Are the advantages of getting an additional 50V worth it ?

The next one is a simple one : is the cap between the gain stage and the buffer a 0.1uf or 0.1pf ? Is the voltage rating the same as the original 4S ?

Firstly I'm running a new 6SN7-SE preamp on 215V where 250V is recommended. It produces nearly the same bias voltage and current drawn. I wouldn't stress 250V is stacks. Don't try and increase it there is no advantage.

Also interstate caps are 0.1uf for a number of different tube amps I've never seen 0.1pf.

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Projects:”Image” - Low Profile 100W Class D amp | ”fiih” - 150W Holton inpsired power amp | "Precision" - 300W Class AB Holton power amp | ”Gothic” - FET Push-Pull headphone amp

That kind of answer both questions at the same time ! Thanks !

This also means that I will be building this preamp soon. I'll take some pictures along the way !

Always interested to see others vision in real life. I always have an image in my mind how something will look once finished. 9/10 times it comes out exactly as I saw it. How it sounds can vary. Thank the Lord it is usually good with only a few duds along a very long road.

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Projects:”Image” - Low Profile 100W Class D amp | ”fiih” - 150W Holton inpsired power amp | "Precision" - 300W Class AB Holton power amp | ”Gothic” - FET Push-Pull headphone amp

I added grid resistors to both tubes, right at the grid pins. 1K at the first tube and 470 Ohms at the cathode follower. This reduced the hiss to an acceptable level. This is something to consider if you have a tight build as I did. It was unusable without these resistors in my case.

I am starting to wonder if I should buy a bigger enclosure. I might also build one myself or use one of the steel hammond enclosures I have here. Anyway I need to order the parts for buffer... and I really like stepped attenuators wich are getting more affordable these days...