Transformer based 6db preamp

[happyrabbit]

My latest diy preamp adventure. -/+ 24v Linear Supply. Sparkoslabs regulators. LSK489 for the jfet’s. 6 layer PCB (s/g/p/p/g/s). Startup / shutdown muting.

Balanced -> LL1676(1:2) -> jfet follower -> 10k ladder -> jfet follower -> dc blocking cap -> RCA

I will post measurements soon.. the in/out are flipped on the case.

 

[happyrabbit]

Input @ +4dbu -> 1k @ 1V

16bit picoscope & NTI balanced signal generator.

 

[Mark Tillotson]

10dB/division is much easier to evaluate than 14dB/division as a vertical scale...

 

[Ken Newton]

Good clean looking construction.

 

[happyrabbit]

Mark,

I have only had the picoscope for a month.. I am no expert on the software. I could not find a way to scale @ 10db/division. Sorry !

KEN,

Thanks ! not super exotic but I find the design enjoyable with my solid state amp.


Dwight

 

[happyrabbit]

Round #2

Transformer configured 1:1. Isolated the PS better plus additional filtering. Removed Jfet v-reg’s and replaced with low pass filters. Waiting on new case. Also integrating Tortuga Audio’s new ldr volume attenuator.

Electronic Stepped Attenuator | ePot.V3 Mini | Tortuga Audio

Dwight

 

[analog_sa]

How does the 1684 compare to the 1676 soundwise?

 

[happyrabbit]

Almost there..

 

[happyrabbit]

Frank sounds absolutely lovely. Still have a little hum with my 97db speakers.. but otherwise it’s a killer preamp.

 

[sgrossklass]

Still have a little hum with my 97db speakers.. but otherwise it’s a killer preamp.

Ever heard of an oxymoron?

Your power amp isn't also IEC Class I by any chance? (If not, does its power xfmr have a shield winding?) With all the work you've put in, it would suck to have a ground loop in the setup.

Also, XLR pin 1 hookup is not AES48 compliant by the looks of it, should go straight to chassis instead. And twisted pair arguably isn't first choice for hooking up RCAs, consider something closer to coaxial wiring.

Not sure I would trust a Picoscope for audio work, I mean, it's a scope and not an audio analyzer. Would suggest rechecking distortion via a decent USB soundcard / audio interface input on a battery-powered laptop. The levels displayed earlier would not be very much to write home about.

 

[happyrabbit]

Thank you for the response. Attached a better pic of the build..

Here is a link to my amp. The Amp’s ground lift switch does remove the hum.

Vision SET 120 Power Amplifier – Audio by Van Alstine

I have located a chassis ground point near the xlr inputs. To be honest , I couldn’t decide which direction to go. It won’t take much to wire it

I have ordered a Mu shielded toroid.

https://www.mouser.com/ProductDetai...w%2bjEpvjHw==&countrycode=US&currencycode=USD

I have attached a link for the pic scope I am using.

16 bit high resolution oscilloscope | Overview

Balanced Signal Generator.

MR-PRO Audio Generator

I will attach the lastest FFT on my next post.

And once again.. thank you for your insight !

Dwight

 

[egellings]

It's beautiful.

 

[happyrabbit]

Updated grounding + Installed shielded transformer w/ screen. I can still hear the hum @ 6’’ without the amp’s ground lift..

 

[marconi118]

post any schema?

 

[sgrossklass]

Here is a link to my amp. The Amp’s ground lift switch does remove the hum.

Vision SET 120 Power Amplifier – Audio by Van Alstine

Looks like legit backyard highend. Unbalanced inputs only, IEC Class I, ground lift switch. Why then do you have a preamp with balanced outputs?

Wait, those are female XLR jacks - i.e. inputs. They're just labeled wrong then, I guess. Oh, you even mentioned that in your first post. Threw me for a bit of a loop there.

You placed the transformers on the input and the output is ground-referenced, right? And ground goes to chassis which is earthed.

I'm sorry to say, you'll have to keep using the ground lift switch in this combination. You have an unbalanced connection between two IEC Class I devices otherwise (those being your preamp and the power amp), which is fundamentally broken as you can hear.

Your project would have made more sense to me if you had had transformer-coupled, floating RCA inputs, XLR outputs, and a power amp with XLR inputs. One of those could have been used to drive something using Hypex nCore or Purifi Class D modules or a solid vintage PA amp like an '80s Yamaha or something.

 

[Bigun]

Couple of questions

Is that a steel chassis, if so where did you get it from ?

Why did you choose ClarityCap ?

Thx

 

[marconi118]

if you could post a schema it would be great
thanks

 

[happyrabbit]

Bigun,

The case is from Protocase Designer | The fastest and easiest way to design, price and build custom electronics enclosures its Aluminum but steel is also available. I like the Clarity CSA'S. Not super expensive.

Marconi118,

Here is the SCH for one side ! Sorry for the delay Pretty simple design. LSK489's sound lovely. They need to be sorted. Lots of low Idds.. Buy a bunch !

Dwight