Thoughts On MAD HD Super Design?

[roadtonowhere08]

Hi All,

I have had my MAD HD Super for a while now, and I am very happy with it. I am not very versed in circuit design for audio, but I am curious by nature. Upon first glance, I gather that it is a simple but effective lower powered circuit. Since I know many here are very skilled in this area, I am curious about your thoughts about the design. I have no intention of replacing it; rather, this is me asking for a layman's explanation for how it works and how it compares to other similarly priced tube amps.

I have attached the User's Manual in case anyone is interested in looking at it. I have never seen it on Dr. Peppard's website, but his regular HD Manual can be downloaded (along with his newer Super design), so I assume it is alright to post.

Mapletree Audio EAR+ HD Super Users' Manual.zip

[dsavitsk]

this is me asking for a layman's explanation for how it works

The last page of the file you uploaded has a pretty thorough description of how the circuit works.

[roadtonowhere08]

I reread that page, and I understood about half of it. I have some thoughts/questions if you or anyone else has the chance:

1. Is it safe to say that a "cathode follower" is the 12B4A tube which acts to lower the impedance coming from the 5751 tube to properly drive the transformers in the audio signal? The transformers are used to be able to properly drive headphones with lower impedances like the 32 ohm Grados. In essence, overall voltage capability is lowered (thus total output capability) in order to lower the output impedance. Correct?

2. The 7 ohm output impedance from the Super is low but not super low, and that might be a reason why it goes especially well with Grados? I remember reading someone here (I think it might be you) say that a slightly higher output impedance gives a more warm presentation to headphones, and that tends to help Grados be more lively and warm. A really low output impedance has great control over the drivers, but it tends to overdamp Grados. Is that correct?

3. The first things to wear out in amps are electrolytic caps. I had Dr. Peppard use Blackgate caps in the power supply (and perhaps elsewhere in the chain), because based on what I read about their construction, they would perhaps last longer than regular electrolytics from Nichicon. Aside from the claims of better sound (I doubt I could pass a DBT, and it is not important to me), is there any merit to my thoughts on this? Regardless of whether or not they last longer, would you happen to know what a typical lifespan is on good quality caps in headphone amp roles? I am assuming about 20 years.

[dsavitsk]

1. Is it safe to say that a "cathode follower" is the 12B4A tube which acts to lower the impedance coming from the 5751 tube to properly drive the transformers in the audio signal? The transformers are used to be able to properly drive headphones with lower impedances like the 32 ohm Grados. In essence, overall voltage capability is lowered (thus total output capability) in order to lower the output impedance. Correct?

A cathode follower, which is what the 12B4 is, has a gain of slightly less than 1, and an output impedance (Zout) of 1/gm. The 12B4 has a gm of around 0.006 S. This means that the Zout from that stage is ~170 Ohms (the white paper you uploaded says 500 Ohms, but this is incorrect).

A transformer is an impedance and voltage conversion device. The transformer used has an impedance ratio of 600:8 or 600:4 depending on the tap used. Since the 4 Ohm tap is used for Grados, this is equivalent to a 4800:32 impedance (or 150:1). This means that a 32 Ohm load on the secondary will appear as a 4800 Ohm load on the primary, and that the 170 Ohm output from the tube will appear as a 170/150 or just over an Ohm. Additionally, because transformers are not perfect devices, the impedance of the copper adds to this, which makes the Zout somewhat higher -- by at least a few Ohms.

To know how much higher, one would need to measure the transformer. However, the quoted 7 Ohm number is based on the incorrectly calculated 500 Ohm Zout of the 12B4 stage and the 8 Ohm tap. It also fails to account for the losses in the transformer. So, the fact that the actual Zout is probably pretty close to that is a bit of a coincidence.

Transformers also transform voltages, and they do this as the square root of the impedance ratio. So, the 150:1 impedance ratio means a ~12.25:1 voltage ratio. Thus, a 12V signal on the grid of the 12B4 will result in less than 1V at the amplifier's output.

The driver stage has a lot of gain. A tube's mu is the gain, and the 12AX7 has a mu of 100. However, inefficiencies in the circuit keep this down -- let's say to 60 or so. This means that a 1V signal on the amplifier's input will be an estimated 60V signal at the grid of the 12B4 and close to 5V on the output. Thus an overall gain of ~5. Additionally, the input will clip at 2Vp-p which is 10Vp-p on the output, or 3.5Vrms (which is what is stated in the document as the power limit).

2. The 7 ohm output impedance from the Super is low but not super low, and that might be a reason why it goes especially well with Grados? I remember reading someone here (I think it might be you) say that a slightly higher output impedance gives a more warm presentation to headphones, and that tends to help Grados be more lively and warm. A really low output impedance has great control over the drivers, but it tends to overdamp Grados. Is that correct?

Sound like the sort of thing I might say.

3. The first things to wear out in amps are electrolytic caps. I had Dr. Peppard use Blackgate caps in the power supply (and perhaps elsewhere in the chain), because based on what I read about their construction, they would perhaps last longer than regular electrolytics from Nichicon. Aside from the claims of better sound (I doubt I could pass a DBT, and it is not important to me), is there any merit to my thoughts on this? Regardless of whether or not they last longer, would you happen to know what a typical lifespan is on good quality caps in headphone amp roles? I am assuming about 20 years.

I would doubt Blackgate caps would be better in this regard, but I really have no idea. 20 years is probably pretty optimistic for any electrolytic caps. There was a pretty nice post a few days ago from Tangent in the DIY section at HF regarding capacitor life expectancy.

http://www.head-fi.org/t/583878/capacitor-lifespan

Edited December 22, 2011 by dsavitsk

[roadtonowhere08]

Wow, dsavitsk! Great reply. I will go over that a few times to get it, but it's very helpful. Thank you