This design started out as request from an audio nut friend. It uses an EF86 as a pre-amp, a 12AX7 as the phase splitter and a pair of Sovtek 6550A beam power pentodes in a class AB output stage. The design makes about 40-50W into 8 ohms, with a THD figure of under 0.5% from 100Hz - 20kHz. Distortion is a little higher below 100Hz due to the output transformers - which are a little under sized due to my friends (and my) budget. A toriodal power transformer was utlized to keep hum noise low. Besides typical left and right level controls, I added a contour control that effectively tilts the frequency response "up" (meaning a bit brighter) or "down" (meaning a bit warmer) centered around 600Hz.
The sound of this amplifier is simply stunning! I'm a skeptic when it comes to audio voodoo magic - but listening some CD's through this amplifier (NAD 1130 pre-amp and my home made ETON clone speakers) revealed stuff I'd never heard before. My wife actually walked in the room thinking I had a friend over because she heard the individual background vocalists so clearly! I'm not going to burden this page with too many words...if you build or buy this amplifier you'll absolutely love it! It has become the one I listen with most often.
Contact me at geoff@bandonelectronics.com if your're interested in this amplifier. I liked it so much I decided not to give it to my friend after all - I built him another one!
The 807 is a classic coke bottle shaped tube primarily intended as an RF amplifier. I decided to take on the challenge of building an audio amplifier out of them. The fact they have a top, external anode connection that would provide a cool look was all I needed to go ahead with the design.
Knowing I was probably going to have run the 807's at reduced gain to tame any high frequency instability issues that may arise I went with a 2-stage preamp design using both halves of an 6SN7 dual triode. A second 6SN7 acts as the phase splitter. The 807's are run in triode mode - by coupling the screens to the plates through a 100 ohm resistor. The plate impedance of the 807 can approach 10K ohms - so the output transformer selection was tricky. I selected a pair of Hammond 1620's - not optimal because they only offer an approximate 7K impedance - but they were within my budget and came close. I added a pair of millimeters and switching to allow measurement of the 807 cathode current to facilitate matching if needed as "matched sets" of 807's are hard to come by. Sadly two 807's rolled off my work bench during the construction of this amplifier and met an untimely end.
This amplifier sounds very open in the mid and treble ranges but may be considered a bit weak in the low end. Paired with a subwoofer you'd never notice! It provides around 25W and offers classic tube sound and unusual looks. The blue front, copper feet and heavy gauge anode wires make it a conversation piece.
Contact me at geoff@bandonelectronics.com if your're interested in this amplifier. I can build this design into any configuration - so you don't have to go with the look I chose if you'd rather have something a little more subdued.
EL84's (and the more generic but identical 6BQ5) are classic staples of tube design. They are pentodes intended for audio power amplification and make there home in numerous guitar amp's, mid-60's hi-fi gear and console sets. Any tube guy worth his weight in solder has at least one design using these tubes in his collection!
This design uses 12AX7's on the input side and a pair of EL84's running class AB on the output. A unique design feature allows the amp to be run in either ultra-linear or triode mode at the turn of a switch. The example shown in the picture uses a separate transformer for the high voltage and filaments - since that's what I had on hand. It is totally conventional, stable and easily reproduced. To go with this classic heritage I wrapped the chassis in oak and finished it with several coats of black lacquer. Brushed aluminum knobs complete the look.
The sound is what you'd expect from EL84's in class AB - open, clean and something you can listen to for hours without fatigue. LP's played through this amplifier sound amazing, but thanks to it's 20W output power it does CD's justice as well. Interstingly - like most tube amp's - it makes compressed audio sources (MP3 etc.) very listenable.
Contact me at geoff@bandonelectronics.com if your're interested in this amplifier. This amplifer will be offered as partial or full kit as well.
I've wanted a ridiculously powerful amplifier for along time and so started what I call the "A" series to create something with 250+ watts output, exemplary specifications
and bullet proof operation. This last requirement proved ellusive - several early "A" series designs went up in smoke! The A8 resolves all of the issues from ealier designs
while still providing in excess of 300W RMS per channel. A unique feature allows the amplifier to run in "bridged" mode where it provides this 300W output, or near class A
with a more modest 75-100W output level.
To achieve such high levels of output power is not trivial. A very robust power supply is needed, and thermal management is critical for long life. This design is more of a
constant voltage style amplifier using a somewhat low split supply of only +/- 35V. High power is achieved by parallel connecting several output devices, which also helps
lower the output impedance and drive more current into the speakers. The deisgn uses no feedback of any kind and yet is unconditionally stable. Frequency response is
DC - 200kHz (-3dB), or can be band limited if desired. Large heatsinks and push-pull speed control fans (speed is fixed, or varies by level or heatsink temp) keep thermals
under control.
In near class A mode things get interesting. Prolonged operation at maximum power gets things hot in a hurry! The protection circuitry will prevent damage but this mode should
be for more initimate listening sessions - not filling the house with sound! Switching between bridged and class-A mode entails a brief moment of silence as the output relays
click open.
The A8 sounds like all solid state amp's - neutral, flat, un-imposing. The massive amount of output power available makes music simply breath taking to hear - clean dynamics,
effortless volume and a sort of richness heard only with very high power amplifiers. While you could cause this amplifier to clip - it's unlikely! The lack of feedback means
TIM / IM distortion is very low, and the amplifiers nearly 3dB of dynamic headroom means it delivers over 600W peak power. Front panel indicators for power supply voltages,
operating mode, protection circuit status and fan speed let you keep tabs on things when pushing the amp hard (like party time - keep the neighbors up!). I added a set of LED
power meters as well to the design example shown in the picture.
Contact me at geoff@bandonelectronics.com if your're interested in this amplifier. This amplifer will be offered as partial or full kit as well.