Hi all, I found this (I think new) company from my country making some amps among them this OTL headphone amp, that by type of schematic it seems a middle ground between the bottlehead and the darkvoice. Nothing new under the sun, I ask an opinion about quality/circuit because it doesn't fully convince me. These are the links:
LESAHiFi | Amplificatori, Preamplificatori, Alimentatori, Valvole
[PASCAL GOLD] PCB DIY KIT Amplificatore Valvolare OTL (Cuffie)/Tube headphones | eBay
LESAHiFi | Amplificatori, Preamplificatori, Alimentatori, Valvole
[PASCAL GOLD] PCB DIY KIT Amplificatore Valvolare OTL (Cuffie)/Tube headphones | eBay
You linked the italian version of the manufacturer web site, the english version may be more useful to forum members LESAHiFi | Amplificatori, Preamplificatori, Alimentatori, Valvole . The company has been registered on late 2017. My first reaction to this product is about the form factor. It may be difficult to fit this board inside a commercial chassis, because the power semiconductors are on both sides of a narrow 115mm PCB, so a custom chassis with heatsink is needed. This will further increase the already high price tag. The filament supply voltage is specified at 6Vac 5A, but then the input DC voltage headroom of the LT1083 regulator will be minimal. I would have used a 7-8VAC transformer instead. Due to the high output impedence specification (80 ohm), a 30 ohm headphone would be a bad fit for this amplifier. At power-up, the output capacitor will charge trough the headphone (again: bad news for low impedence headphones). A timed relay and a zener protection on the headphone output may be a useful addition, but the circuit is pretty basic and it should work as is.
Yes, I agree on the not optimal construction and the need of proper heatsinks requiring a rather big case for an headphone amp. Even if they put Mundorf caps, overall it seems they aimed to put components to make it "cool" and, as for me, a simplier circuit could achieve the same/or better results. In any case yes, it cannot drive such a low impedance headphones as they claim.
I notice that the OTL headphone amp is not directly coupled. This forces the cathode bias resistor on the 6080 to be a pretty low value (1K) and when you use high impedance headphones, much of the available AC output current will be cooked away in that resistor instead of being delivered to the headphones.
I wouldn't feel all that confidently about using a PC board mount socket and a 6080. That's a lot of heat being generated and dumped into the PC board.
I wouldn't feel all that confidently about using a PC board mount socket and a 6080. That's a lot of heat being generated and dumped into the PC board.
The 1K cathode resistor is on the schematic of the now discontinued Pascal amplifier listed on the first post. The current design (Pascal Gold) is referenced on the ebay listing and on my post. It does use a CCS. Modern circuit boards work fine with heat, but the Epcos capacitors are very close to the heatsinks and the vintage blue Philips capacitor in the picture is probably rated at 85 degrees. I believe that this basic circuit does work at best when it is kept simple. By the time you add CCS, dual stabilized power supply, dual output, beefy chassis with heatsink and premium components, the total build cost has risen to a point that a different design becomes a better option.
I don't know. Call me silly, but I'd at least add a 2.2 MΩ resistor from B+ to the grid-to-ground pull-down resistor facing each 6080's triode. This'd pull up the grid by about ¹⁄₁₁ of the B+ supply, giving greater headroom opportunities to the cathode-follower output.
It also would give the nice potential 'win' to simply adding more 6080 sections “in parallel” to increase total available power, and dynamics. Not all headphones are created equal!
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
It also would give the nice potential 'win' to simply adding more 6080 sections “in parallel” to increase total available power, and dynamics. Not all headphones are created equal!
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
Yes, It was that I thought seeing the circuit...also adding AC coupling as Frank Berry observed has its limitation because of the amount of current request.
One marginal note about of these (small) companies of my country, apart this there are also some "excellences" such as Megahertz from Naples, and a new one soon discovered Viva Audio from Vicenza, where I think to go for a listening the first week of march.
If we need efficiency, then generally an OTL amp is not efficient.
That is why they build both OTL amps, and amps with output transformers . . .
some amps go for efficiency, and some want to have an OTL amp.
Matching cathode followers quiescent can be done by using individual self biasing for each cathode, and using individual output capacitors for each cathode, then tying them together at the output connector.
Some problems are only solved by having increased complexity and parts.
That is why they build both OTL amps, and amps with output transformers . . .
some amps go for efficiency, and some want to have an OTL amp.
Matching cathode followers quiescent can be done by using individual self biasing for each cathode, and using individual output capacitors for each cathode, then tying them together at the output connector.
Some problems are only solved by having increased complexity and parts.
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